Mutation (Jordan algebra)

In mathematics, a mutation, also called a homotope, of a unital Jordan algebra is a new Jordan algebra defined by a given element of the Jordan algebra. The mutation has a unit if and only if the given element is invertible, in which case the mutation is called a proper mutation or an isotope. Mutations were first introduced by Max Koecher in his Jordan algebraic approach to Hermitian symmetric spaces and bounded symmetric domains of tube type. Their functorial properties allow an explicit construction of the corresponding Hermitian symmetric space of compact type as a compactification of a finite-dimensional complex semisimple Jordan algebra. The automorphism group of the compactification becomes a complex subgroup, the complexification of its maximal compact subgroup. Both groups act transitively on the compactification. The theory has been extended to cover all Hermitian symmetric spaces using the theory of Jordan pairs or Jordan triple systems. Koecher obtained the results in the more general case directly from the Jordan algebra case using the fact that only Jordan pairs associated with period two automorphisms of Jordan algebras are required.

Definitions

Let A be a unital Jordan algebra over a field k of characteristic ≠ 2.^[1] For a in A define the Jordan multiplication operator on A by

and the quadratic representation Q(a) by

It satisfies

the fundamental identity

the commutation or homotopy identity

where

In particular if a or b is invertible then

It follows that A with the operations Q and R and the identity element defines a quadratic Jordan algebra, where a quadratic Jordan algebra consists of a vector space A with a distinguished element 1 and a quadratic map of A into endomorphisms of A, a ↦ Q(a), satisfying the conditions:

• Q(1) = id
• Q(Q(a)b) = Q(a)Q(b)Q(a) ("fundamental identity")
• Q(a)R(b,a) = R(a,b)Q(a) ("commutation or homotopy identity"), where R(a,b)c = (Q(a + c) − Q(a) − Q(c))b

The Jordan triple product is defined by

so that

There are also the formulas

For y in A the mutation A^y is defined to the vector space A with multiplication

If Q(y) is invertible, the mutual is called a proper mutation or isotope.

Quadratic Jordan algebras

Let A be a quadratic Jordan algebra over a field k of characteristic ≠ 2. Following Jacobson (1969), a linear Jordan algebra structure can be associated with A such that, if L(a) is Jordan multiplication, then the quadratic structure is given by Q(a) = 2L(a)² − L(a²).

Firstly the axiom Q(a)R(b,a) = R(a,b)Q(a) can be strengthened to

Indeed, applied to c, the first two terms give

Switching b and c then gives

Now let

Replacing b by a and a by 1 in the identity above gives

In particular

The Jordan product is given by

so that

The formula above shows that 1 is an identity. Defining a² by a∘a = Q(a)1, the only remaining condition to be verified is the Jordan identity

In the fundamental identity

Replace a by a + t1, set b = 1 and compare the coefficients of t² on both sides:

Setting b = 1 in the second axiom gives

and therefore L(a) must commute with L(a²).

Inverses

Let A be a unital Jordan algebra over a field k of characteristic ≠ 2. An element a in a unital Jordan algebra A is said to be invertible if there is an element b such that ab = 1 and a²b = a.^[2]

Properties.^[3]

If ab = 1 and a²b = a, then Q(a)b = 2a(ab) − (a²)b = a. The Jordan identity [L(x),L(x²)] = 0 can be polarized by replacing x by x + ty and taking the coefficient of t. This gives

Taking x = a or b and y = b or a shows that L(a²) commutes with L(b) and L(b²) commutes with L(a). Hence (b²)(a²) = 1. Applying L(b) gives b²a = b. Hence Q(a)b² = 1. Conversely if Q(a)b = a and Q(a)b² = 1, then the second relation gives Q(a)Q(b)² Q(a) = I. So both Q(a) and Q(b) are invertible. The first gives Q(a)Q(b)Q(a) = Q(a) so that Q(a) and Q(b) are each other's inverses. Since L(b) commutes with Q(b) it commutes with its inverse Q(a). Similarly L(a) commutes with Q(b). So (a²)b = L(b)a² = Q(a)b = a and ab = L(b)Q(a)b= Q(a)Q(b)1= 1.

Indeed, if a is invertible then the above implies Q(a) is invertible with inverse Q(b). Any inverse b satisfies Q(a)b = a, so b = Q(a)⁻¹a. Conversely if Q(a) is invertible let b = Q(a)⁻¹a. Then Q(a)b = a. The fundamental identity then implies that Q(b) and Q(a) are each other's inverses so that Q(a)b² = Q(a)Q(b)1=1.

This follows from the formula a⁻¹ = Q(a)⁻¹a.

Suppose that Q(a)c = 1. Then by the fundamental identity Q(a) is invertible, so a is invertible.

This is an immediate consequence of the fundamental identity and the fact that STS is invertible if and only S and T are invertible.

In the commutation identity Q(a)R(b,a) = Q(Q(a)b,a), set b = c² with c = a⁻¹. Then Q(a)b = 1 and Q(1,a) = L(a). Since L(a) commutes with L(c²), R(b,a) = L(c) = L(a⁻¹).

If L(a) and L(b) commute, then ba = 1 implies b(a²) = a. Conversely suppose that a is invertible with inverse b. Then ab = 1. Morevoer L(b) commutes with Q(b) and hence its inverse Q(a). So it commutes with L(a) = Q(a)L(b).

The algebra k[a] is commutative and associative, so if b is an inverse there ab =1 and a²b = a. Conversely Q(a) leaves k[a] invariant. So if it is bijective on A it is bijective there. Thus a⁻¹ = Q(a)⁻¹a lies in k[a].

Elementary properties of proper mutations

In fact ^[4] multiplication in the algebra A^y is given by

so by definition is commutative. It follows that

with

If e satisfies a ∘ e = a, then taking a = 1 gives

Taking a = e gives

so that L(y) and L(e) commute. Hence y is invertible and e = y⁻¹.

Now for y invertible set

Then

Moreover,

Finally

since

Hence

Thus (A,Q^y,y⁻¹) is a unital quadratic Jordan algebra. It therefore corresponds to a linear Jordan algebra with the associated Jordan multiplication operator M(a) given by

This shows that the operators L_y(a) satisfy the Jordan identity so that the proper mutation or isotope A^y is a unital Jordan algebra. The correspondence with quadratic Jordan algebras shows that its quadratic representation is given by Q_y.

Nonunital mutations

The definition of mutation also applies to non-invertible elements y. If A is finite-dimensional over R or C, invertible elements a in A are dense, since invertibility is equivalent to the condition that det Q(a) ≠ 0. So by continuity the Jordan identity for proper mutations implies the Jordan identity for arbitrary mutations. In general the Jordan identity can be deduced from Macdonald's theorem for Jordan algebras because it involves only two elements of the Jordan algebra. Alternatively, the Jordan identity can be deduced by realizing the mutation inside a unital quadratic algebra.^[5]

For a in A define a quadratic structure on A₁ = A ⊕ k by

It can then be verified that (A₁, Q₁, 1) is a unital quadratic Jordan algebra. The unital Jordan algebra to which it corresponds has A^y as an ideal, so that in particular A^y satisfies the Jordan identity. The identities for a unital quadratic Jordan algebra follow from the following compatibility properties of the quadratic map Q_y(a) = Q(a)Q(y) and the squaring map S_y(a) = Q(a)y:

• R_y(a,a) = L_y(S_y(a)).
• [Q_y(a),L_y(a)] = 0.
• Q_y(a)S_y(a) = S_y(S_y(a)).
• Q_y∘ S_y = S_y ∘ Q_y.
• Q_y(a) Q_y(b) S_y(a) = S_y(Q_y(a)b).
• Q_y(Q_y(a)b) = Q_y(a) Q_y(b) Q_y(a).

Hua's identity

Let A be a unital Jordan algebra. If a, b and a – b are invertible, then Hua's identity holds:^[6]

In particular if x and 1 – x are invertible, then so too is 1 – x⁻¹ with

To prove the identity for x, set y = (1 – x)⁻¹. Then L(y) = Q(1 – x)⁻¹L(1 – x). Thus L(y) commutes with L(x) and Q(x). Since Q(y) = Q(1 – x)⁻¹, it also commutes with L(x) and Q(x). Since L(x⁻¹) = Q(x)⁻¹L(x), L(y) also commutes with L(x⁻¹) and Q(x⁻¹).

It follows that (x⁻¹ – 1)xy =(1 – x) y = 1. Moreover, y – 1 = xy since (1 – x)y = 1. So L(xy) commutes with L(x) and hence L(x⁻¹ – 1). Thus 1 – x⁻¹ has inverse 1 – y.

Now let A^a be the mutation of A defined by a. The identity element of A^a is a⁻¹. Moreover, an invertible element c in A is also invertible in A^a with inverse Q(a)⁻¹ c⁻¹.

Let x = Q(a)⁻¹b in A^a. It is invertible in A, as is a⁻¹ – Q(a)⁻¹b = Q(a)⁻¹(a – b). So by the special case of Hua's identity for x in A^a

Bergman operator

If A is a unital Jordan algebra, the Bergman operator is defined for a, b in A by^[7]

If a is invertible then

while if b is invertible then

In fact if a is invertible

Q(a)Q(a⁻¹ − b) = Q(a)[Q(a⁻¹ − 2Q(a⁻¹,b) + Q(b)]=I − 2Q(a)Q(a⁻¹,b) + Q(a)Q(b)=I − R(a,b) + Q(a)Q(b)

and similarly if b is invertible.

More generally the Bergman operator satisfies a version of the commutation or homotopy identity:

and a version of the fundamental identity:

There is also a third more technical identity:

Quasi-invertibility

Let A be a finite-dimensional unital Jordan algebra over a field k of characteristic ≠ 2.^[8] For a pair (a,b) with a and a⁻¹ − b invertible define

In this case the Bergman operator B(a,b) = Q(a)Q(a⁻¹ − b) defines an invertible operator on A and

In fact

Moreover, by definition a⁻¹ − b − c is invertible if and only if (a^b)⁻¹ − c is invertible. In that case

Indeed,

The assumption that a be invertible can be dropped since a^b can be defined only supposing that the Bergman operator B(a,b) is invertible. The pair (a,b) is then said to be quasi-invertible. In that case a^b is defined by the formula

If B(a,b) is invertible, then B(a,b)c = 1 for some c. The fundamental identity implies that B(a,b)Q(c)B(b,a) = I. So by finite-dimensionality B(b,a) is invertible. Thus (a,b) is invertible if and only if (b,a) is invertible and in this case

In fact

B(a,b)(a + Q(a)b^a) = a − 2R(a,b)a + Q(a)Q(b)a + Q(a)(b − Q(b)a) = a − Q(a)b,

so the formula follows by applying B(a,b)⁻¹ to both sides.

As before (a,b+c) is quasi-invertible if and only if (a^b,c) is quasi-invertible; and in that case

If k = R or C, this would follow by continuity from the special case where a and a⁻¹ − b were invertible. In general the proof requires four identities for the Bergman operator:

In fact applying Q to the identity B(a,b)a^b = a − Q(a)b yields

The first identity follows by cancelling B(a,b) and B(b,a). The second identity follows by similar cancellation in

B(a,b)Q(a^b,c)B(b,a) = Q(B(a,b)a^b,B(a,b)c) = Q(a − Q(a)b,B(a,b)c) = B(a,b)(Q(a,c) − R(c,b)Q(a)) = (Q(a,c) − Q(a)R(b,c))B(b,a).

The third identity follows by applying the second identity to an element d and then switching the roles of c and d. The fourth follows because

B(a,b)B(a^b,c) = B(a,b)(I − R(a^b,c) + Q(a^b)Q(c)) = I − R(a,b + c) + Q(a) Q(b + c) = B(a,b+c).

In fact (a,b) is quasi-invertible if and only if a is quasi-invertible in the mutation A^b. Since this mutation might not necessarily unital this means that when an identity is adjoint 1 − a becomes invertible in A^b ⊕ k1. This condition can be expressed as follows without mentioning the mutation or homotope:

In fact if (a,b) is quasi-invertible, then c = a^b satisfies the first identity by definition. The second follows because B(a,b)Q(a^b) = Q(a). Conversely the conditions state that in A^b ⊕ k1 the conditions imply that 1 + c is the inverse of 1 − a. On the other hand, ( 1 − a) ∘ x = B(a,b)x for x in A^b. Hence B(a,b) is invertible.

Equivalence relation

Let A be a finite-dimensional unital Jordan algebra over a field k of characteristic ≠ 2.^[9] Two pairs (a_i,b_i) with a_i invertible are said to be equivalent if (a₁)⁻¹ − b₁ + b₂ is invertible and a₂ = (a₁)^{b₁ − b₂}.

This is an equivalence relation, since if a is invertible a⁰ = a so that a pair (a,b) is equivalent to itself. It is symmetric since from the definition a₁ = (a₂)^{b₂ − b₁}. It is transitive. For suppose that (a₃,b₃) is a third pair with (a₂)⁻¹ − b₂ + b₃ invertible and a₃ = (a₂)^{b₂ − b₃}. From the above

is invertible and

As for quasi-invertibility, this definition can be extended to the case where a and a⁻¹ − b are not assumed to be invertible.

Two pairs (a_i,b_i) are said to be equivalent if (a₁, b₁ − b₂) is quasi-invertible and a₂ = (a₁)^{b₁ − b₂}. When k = R or C, the fact that this more general definition also gives an equivalence relation can deduced from the invertible case by continuity. For general k, it can also be verified directly:

• The relation is reflexive since (a,0) is quasi-invertible and a⁰ = a.
• The relation is symmetric, since a₁ = (a₂)^{b₂ − b₁}.
• The relation is transitive. For suppose that (a₃,b₃) is a third pair with (a₂, b₂ − b₃) quasi-invertible and a₃ = (a₂)^{b₂ − b₃}. In this case

so that (a₁,b₁ − b₃) is quasi-invertible with

The equivalence class of (a,b) is denoted by (a:b).

Structure groups

Let A be a finite-dimensional complex semisimple unital Jordan algebra. If T is an operator on A, let T^t be its transpose with respect to the trace form. Thus L(a)^t = L(a), Q(a)^t = Q(a), R(a,b)^t = R(b,a) and B(a,b)^t = B(b,a). The structure group of A consists of g in GL(A) such that

They form a group Γ(A). The automorphism group Aut A of A consists of invertible complex linear operators g such that L(ga) = gL(a)g⁻¹ and g1 = 1. Since an automorphism g preserves the trace form, g⁻¹ = g^t.

• The structure group is closed under taking transposes g ↦ g^t and adjoints g ↦ g*.
• The structure group contains the automorphism group. The automorphism group can be identified with the stabilizer of 1 in the structure group.
• If a is invertible, Q(a) lies in the structure group.
• If g is in the structure group and a is invertible, ga is also invertible with (ga)⁻¹ = (g^t)⁻¹a⁻¹.
• The structure group Γ(A) acts transitively on the set of invertible elements in A.
• Every g in Γ(A) has the form g = h Q(a) with h an automorphism and a invertible.

The complex Jordan algebra A is the complexification of a real Euclidean Jordan algebra E, for which the trace form defines an inner product. There is an associated involution a ↦ a* on A which gives rise to a complex inner product on A. The unitary structure group Γ_u(A) is the subgroup of Γ(A) consisting of unitary operators, so that Γ_u(A) = Γ(A) ∩ U(A). The identity component of Γ_u(A) is denoted by K. It is a connected closed subgroup of U(A).

• The stabilizer of 1 in Γ_u(A) is Aut E.
• Every g in Γ_u(A) has the form g = h Q(u) with h in Aut E and u invertible in A with u* = u⁻¹.
• Γ(A) is the complexification of Γ_u(A).
• The set S of invertible elements u in A such that u* = u⁻¹ can be characterized equivalently either as those u for which L(u) is a normal operator with uu* = 1 or as those u of the form exp ia for some a in E. In particular S is connected.
• The identity component of Γ_u(A) acts transitively on S
• Given a Jordan frame (e_i) and v in A, there is an operator u in the identity component of Γ_u(A) such that uv = ∑ α_i e_i with α_i ≥ 0. If v is invertible, then α_i > 0.

The structure group Γ(A) acts naturally on X.^[10] For g in Γ(A), set

Then (x,y) is quasi-invertible if and only if (gx,(g^t)⁻¹y) is quasi-invertible and

In fact the covariance relations for g with Q and the inverse imply that

if x is invertible and so everywhere by density. In turn this implies the relation for the quasi-inverse. If a is invertible then Q(a) lies in Γ(A) and if (a,b) is quasi-invertible B(a,b) lies in Γ(A). So both types of operators act on X.

The defining relations for the structure group show that it is a closed subgroup of of GL(A). Since Q(e^a) = e^2L(a), the corresponding complex Lie algebra contains the operators L(a). The commutators [L(a),L(b)] span the complex Lie algebra of derivations of A. The operators R(a,b) = [L(a),L(b)] + L(ab) span and satisfy R(a,b)^t = R(b,a) and [R(a,b),R(c,d)]=R(R(a,b)c,d) − R(c,R(b,a)d).

Geometric properties of quotient space

Let A be a finite-dimensional complex unital Jordan algebra which is semisimple, i.e. the trace form Tr L(ab) is non-degenerate. Let X be the quotient of A×A by the equivalence relation. Let X_b be the subset of X of classes (a:b). The map φ_b:X_b → A, (a:b) ↦ a is injective. A subset U of X is defined to be open if and only if U ∩ X_b is open for all b.

The transition maps of the atlas with charts φ_b are given by

and are injective and holomorphic since

with derivative

This defines the structure of a complex manifold on X because φ_dc ∘ φ_cb = φ_db on φ_b(X_b ∩ X_c ∩ X_d).

Indeed, all the polynomial functions p_i(b) = det B(a_i,b_i − b) are non-trivial since p_i(b_i) = 1. Therefore, there is a b such that p_i(b) ≠ 0 for all i, which is precisely the criterion for (a_i:b_i) to lie in X_b.

Loos (1977) uses the Bergman operators to construct an explicit biholomorphism between X and a closed smooth algebraic subvariety of complex projective space.^[11] This implies in particular that X is compact. There is a more direct proof of compactness using symmetry groups.

Given a Jordan frame (e_i) in E, for every a in A there is a k in U = Γ_u(A) such that a=k(∑ α_i e_i) with α_i ≥ 0 (and α_i > 0 if a is invertible). In fact, if (a,b) is in X then it is equivalent to k(c,d) with c and d in the unital Jordan subalgebra A_e = ⊕ Ce_i, which is the complexification of E_e = ⊕ Re_i. Let Z be the complex manifold constructed for A_e. Because A_e is a direct sum of copies of C, Z is just a product of Riemann spheres, one for each e_i. In particular it is compact. There is a natural map of Z into X which is continuous. Let Y be the image of Z. It is compact and therefore coincides with the closure of Y₀ = A_e ⊂ A = X₀. The set U⋅Y is the continuous image of the compact set U × Y. It is therefore compact. On the other hand, U⋅Y₀ = X₀, so it contains a dense subset of X and must therefore coincide with X. So X is compact.

The above argument shows that every (a,b) in X is equivalent to k(c,d) with c and d in A_e and k in Γ_u(A). The mapping of Z into X is in fact an embedding. This is a consequence of (x,y) being quasi-invertible in A_e if and only if it is quasi-invertible in A. Indeed, if B(x,y) is injective on A, its restriction to A_e is also injective. Conversely, the two equations for the quasi-inverse in A_e imply that it is also a quasi-inverse in A.

Möbius transformations

Let A be a finite-dimensional complex semisimple unital Jordan algebra. The group SL(2,C) acts by Möbius transformation on the Riemann sphere C ∪ {∞}, the one-point compactification of C. If g in SL(2,C) is given by the matrix

then

There is a generalization of this action of SL(2,C) to A and its compactification X. In order to define this action, note that SL(2,C) is generated by the three subgroups of lower and upper unitriangular matrices and the diagonal matrices. It is also generated by the lower (or upper) unitriangular matrices, the diagonal matrices and the matrix

The matrix J corresponds to the Möbius transformation j(z) = −z⁻¹ and can be written

The Möbius transformations fixing ∞ are just the upper triangular matrices. If g does not fix ∞, it sends ∞ to a finite point a. But then g can be composed with an upper unitriangular to send a to 0 and then with J to send 0 to infinity.

For an element a of A, the action of g in SL(2,C) is defined by the same formula

This defines an element of C[a] provided that γa + δ1 is invertible in A. The action is thus defined everywhere on A if g is upper triangular. On the other hand, the action on X is simple to define for lower triangular matrices.^[12]

• For diagonal matrices g with diagonal entries α and α⁻¹, g(a,b) = (α²a, α⁻²b) is a well-defined holomorphic action on A² which passes to the quotient X. On X₀ = A it agrees with the Möbius action.
• For lower unitriangular matrices, with off-diagonal parameter γ, define g(a,b) = (a,b − γ1). Again this is holomorphic on A² and passes to the quotient X. When b = 0 and γ ≠ 0,

if γa + 1 is invertible, so this is an extension of the Möbius action.

• For upper unitriangular matrices, with off-diagonal parameter β, the action on X₀ = (A:0) is defined by g(a,0) = (a + β1). Loos (1977) showed that this defined a complex one-parameter flow on A. The corresponding holomorphic complex vector field extended to X, so that the action on the compact complex manifold X could be defined by the associated complex flow. A simpler method is to note that the operator J can be implemented directly using its intertwining relations with the unitary structure group.

In fact on the invertible elements in A, the operator j(a) = −a⁻¹ satisfies j(ga) = (g^t)⁻¹j(a). To define a biholomorphism j on X such that j ∘ g = (g^t)⁻¹ ∘ j, it is enough to define these for (a:b) in some suitable orbit of Γ(A) or Γ_u(A). On the other hand, as indicated above, given a Jordan frame (e_i) in E, for every a in A there is a k in U = Γ_u(A) such that a=k(∑ α_i e_i) with α_i ≥ 0.

The computation of j in the associative commutative algebra A_e is straightforward since it is a direct product. For c = ∑ α_i e_i and d = ∑ β_i e_i, the Bergman operator on A_e has determinant det B(c,d) = ∏(1 − α_iβ_i)². In particular det B(c,d − λ) ≠ 0 for some λ ≠ 0. So that (c,d) is equivalent to (x,λ). Let μ = −λ⁻¹. On A, for a dense set of a, the pair (a,λ) is equivalent to (b,0) with b invertible. Then (−b⁻¹,0) is equivalent to (μ − μ²a,μ). Since a ↦ μ − μ²a is holomorphic it follows that j has a unique continuous extension to X such that j ∘ g = (g^t)⁻¹ ∘ j for g in Γ(A), the extension is holomorphic and for λ ≠ 0, μ = −λ⁻¹

The holomorphic transformations corresponding to upper unitriangular matrices can be defined using the fact that they are the conjugates by J of lower unitriangular matrices, for which the action is already known. A direct algebraic construction is given in Dineen, Mackey & Mellon (1999).

This action of SL(2,C) is compatible with inclusions. More generally if e₁, ..., e_m is a Jordan frame, there is an action of SL(2,C)^m on A_e which extends to A. If c = ∑ γ_ie_i and b = ∑ β_ie_i, then S(c) and T(b) give the action of the product of the lower and upper unitriangular matrices. If a = ∑ α_ie_i is invertible, the corresponding product of diagonal matrices act as W = Q(a).^[13] In particular the diagonal matrices give an action of (C*)^m and T^m.

Holomorphic symmetry group

Let A be a finite-dimensional complex semisimple unital Jordan algebra. There is a transitive holomorphic action of a complex matrix group G on the compact complex manifold X. Koecher (1967) described G analogously to SL(2,C) in terms of generators and relations. G acts on the corresponding finite-dimensional Lie algebra of holomorphic vector fields restricted to X₀ = A, so that G is realized as a closed matrix group. It is the complexification of a compact Lie group without center, so a semisimple algebraic group. The identity component H of the compact group acts transitively on X, so that X can be identified as a Hermitian symmetric space of compact type.^[14]

The group G is generated by three types of holomorphic transformation on X:

• Operators W corresponding to elements W in Γ(A) given by W(a,b) = (Wa, (W^t)⁻¹b). These were already described above. On X₀ = A, they are given by a ↦ Wa.
• Operators S_c defined by S_c(a,b) = (a,b + c). These are the analogue of lower unitriangular matrices and form a subgroup isomorphic to the additive group of A, with the given parametrization. Again these act holomorphically on A² and the action passes to the quotient X. On A the action is given by a ↦ a^c if (a,c) is quasi-invertible.
• The transformation j corresponding to J in SL(2,C). It was constructed above as part of the action of PSL(2,C) = SL(2,C)/{±I} on X. On invertible elements in A it is given by a ↦ −a⁻¹.

The operators W normalize the group of operators S_c. Similarly the operator j normalizes the structure group, j ∘ W = (W^t)⁻¹ ∘ j. The operators T_c = j ∘ S_−c ∘ j also form a group of holomorphic transformations isomorphic to the additive group of A. They generalize the upper unitriangular subgroup of SL(2,C). This group is normalized by the operators W of the structure group. The operator T_c acts on A as a ↦ a + c. If c is a scalar the operators S_c and T_c coincide with the operators corresponding to lower and upper unitriangular matrices in SL(2,C). Accordingly, there is a relation j = S₁ ∘ T₁ ∘ S₁ and PSL(2,C) is a subgroup of G. Loos (1977) defines the operators T_c in terms of the flow associated to a holomorphic vector field on X, while Dineen, Mackey & Mellon (1999) give a direct algebraic description.

Indeed, S_bT_a(0:0) = (a:b).

Let G₋₁ and G₊₁ be the complex Abelian groups formed by the symmetries T_c and S_c respectively. Let G₀ = Γ(A).

The two expressions for G are equivalent as follows by conjugating by j.

For a invertible, Hua's identity can be rewritten

Moreover, j = S₁ ∘ T₁ ∘ S₁ and S_c = j ∘ T_−c ∘ j.^[15]

The convariance relations show that the elements of G fall into sets G₀G₁, G₀G₁jG₁, G₀G₁jG₁jG₁, G₀G₁jG₁jG₁jG₁. ... The first expression for G follows once it is established that no new elements appear in the fourth or subsequent sets. For this it suffices to show that^[16]

j ∘ G₁ ∘j ∘ G₁ ∘j ⊆ G₀ G₁ ∘j ∘ G₁ ∘ j ∘ G₁.

For then if there are three or more occurrences of j, the number can be recursively reduced to two. Given a, b in A, pick λ ≠ 0 so that c = a − λ and d = b − λ⁻¹ are invertible. Then

which lies in G₀G₁ ∘ j ∘ G₁ ∘ j ∘ G₁.

It suffices to check that if S_aT_b(0:0) = (0:0), then b = 0. If so (b:0) = (0: −a) = (0:0), so b = 0.

Exchange relations

For a invertible, Hua's identity can be rewritten

Since j = S₁ ∘ T₁ ∘ S₁, the operators Q(a) belong to the group generated by G_±1.^[17]

For quasi-invertible pairs (a,b), there are the "exchange relations"^[18]

This identity shows that B(a,b) is in the group generated by G_±1. Taking inverses, it is equivalent to the identity T_aS_b = S_b^aB(a,b)T_a^b.

To prove the exchange relations, it suffices to check that it valid when applied to points the dense set of points (c:0) in X for which (a+c,b) is quasi-invertible. It then reduces to the identity:

In fact, if (a,b) is quasi-invertible, then (a + c,b) is quasi-invertible if and only if (c,b^a) is quasi-invertible. This follows because (x,y) is quasi-invertible if and only if (y,x) is. Moreover, the above formula holds in this case.

For the proof, two more identities are required:

The first follows from a previous identity by applying the transpose. For the second, because of the transpose, it suffices to prove the first equality. Setting c = b − Q(b)a in the identity B(a,b)R(a^b,c) = R(a,c) − Q(a)Q(b,c) yields

B(a,b)R(a^b,b − Q(b)c) = B(a,b)R(a,b),

so the identity follows by cancelling B(a,b).

To prove the formula, the relations (a + c)^b = B(a,c)⁻¹(a + c − Q(a + c)b) and a^b + B(a,b)⁻¹c^(ba) = B(a + c,b)⁻¹(B(c,b^a) (a − Q(a)b) + c − Q(c)b^a) show that it is enough to prove that

a + c − Q(a + c)b = B(c,b^a) (a − Q(a)b) + c − Q(c)b^a.

Indeed, B(c,b^a) (a − Q(a)b) + c − Q(c)b^a = a + c − Q(a)b + 2R(c,b^a)(a − Q(a)b) − Q(c)[ b^a − Q(b^a)(a − Q(a)b)]. On the other hand, 2R(c,b^a)(a − Q(a)b) = 2R(c,a − Q(a)b)b^a = R(a,b)c = 2Q(a,c)b and b^a − Q(b^a)(a − Q(a)b) = b^a − Q(b)B(a,b)⁻¹(a − Q(a)b) = b^a − Q(b)a^b = b. So B(c,b^a) (a − Q(a)b) + c − Q(c)b^a = a + c − Q(a)b − 2Q(a,c)b − Q(c)b = a + c − Q(a + c)b.

Now set Ω = G₊₁G₀G₋₁. Then the exchange relations imply that S_b T_a lies in Ω if and only if (a,b) is quasi-invertible; and that g lies in Ω if and only if g(0:0) is in X₀.^[19]

In fact if S_b T_a lies in Ω, then (a,b) is equivalent to (x,0), so it a quasi-invertible pair; the converse follows from the exchange relations. Clearly Ω(0:0) = G₁(0:0) = X₀. The converse follows from G = G₋₁G₁ G₀G₋₁ and the criterion for S_b T_a to lie in Ω.

Lie algebra of holomorphic vector fields

The compact complex manifold X is modelled on the space A. The derivatives of the transition maps describe the tangent bundle through holomorphic transition functions F_bc:X_b ∩ X_c → GL(A). These are given by F_bc(a,b) = B(a,b − c), so the structure group of the corresponding principal fiber bundle reduces to Γ(A), the structure group of A.^[20] The corresponding holomorphic vector bundle with fibre A is the tangent bundle of the complex manifold X. Its holomorphic sections are just holomorphic vector fields on X. They can be determined directly using the fact that they must be invariant under the natural adjoint action of the known holomorphic symmetries of X. They form a finite-dimensional complex semisimple Lie algebra. The restriction of these vector fields to X₀ can be described explicitly. A direct consequence of this description is that the Lie algebra is three-graded and that the group of holomorphic symmetries of X, described by generators and relations in Koecher (1967) and Loos (1979), is a complex linear semisimple algebraic group that coincides with the group of biholomorphisms of X.

The Lie algebras of the three subgroups of holomorphic automorphisms of X give rise to linear spaces of holomorphic vector fields on X and hence X₀ = A.

• The structure group Γ(A) has Lie algebra spanned by the operators R(x,y). These define a complex Lie algebra of linear vector fields a ↦ R(x,y)a on A.
• The translation operators act on A as T_c(a) = a + c. The corresponding one-parameter subgroups are given by T_tc and correspond to the constant vector fields a ↦ c. These give an Abelian Lie algebra of vector fields on A.
• The operators S_c defined on X by S_c(a,b) = (a,b − c). The corresponding one-parameter groups S_tc define quadratic vector fields a ↦ Q(a)c on A. These give an Abelian Lie algebra of vector fields on A.

Let

Then, defining for i ≠ −1, 0, 1, forms a complex Lie algebra with

This gives the structure of a 3-graded Lie algebra. For elements (a,T,b) in , the Lie bracket is given by

The group PSL(2,C) of Möbius transformations of X normalizes the Lie algebra . The transformation j(z) = −z⁻¹ corresponding to the Weyl group element J induces the involutive automorphism σ given by

More generally the action of a Möbius transformation

can be described explicitly. In terms of generators diagonal matrices act as

upper unitriangular matrices act as

and lower unitriangular matrices act as

This can be written uniformly in matrix notation as

In particular the grading corresponds to the action of the diagonal subgroup of SL(2,C), even with |α| = 1, so a copy of T.

The Killing form is given by

where β(T₁,T₂) is the symmetric bilinear form defined by

with the bilinear form (a,b) corresponding to the trace form: (a,b) = Tr L(ab).

More generally the generators of the group G act by automorphisms on as

• 
• 
• 
• 

The nondegeneracy of the Killing form is immediate from the explicit formula. By Cartan's criterion, is semisimple. In the next section the group G is realized as the complexification of a connected compact Lie group H with trivial center, so semisimple. This gives a direct means to verify semisimplicity. The group H also acts transitively on X.

To prove that exhausts the holomorphic vector fields on X, note the group T acts on holomorphic vector fields. The restriction of such a vector field to X₀ = A gives a holomorphic map of A into A. The power series expansion around 0 is a convergent sum of homogeneous parts of degree m ≥ 0. The action of T scales the part of degree m by α^{2m − 2}. By taking Fourier coefficients with respect to T, the part of degree m is also a holomorphic vector field. Since conjugation by J gives the inverse on T, it follows that the only possible degrees are 0, 1 and 2. Degree 0 is accounted for by the constant fields. Since conjugation by J interchanges degree 0 and degree 2, it follows that account for all these holomorphic vector fields. Any further holomorphic vector field would have to appear in degree 1 and so would have the form a ↦ Ma for some M in End A. Conjugation by J would give another such map N. Moreover, e^tM(a,0,0)= (e^tMa,0,0). But then

Set U_t = e^tM and V_t = e^tB. Then

It follows that U_t lies in Γ(A) for all t and hence that M lies in . So is exactly the space of holomorphic vector fields on X.

Compact real form

Suppose g = WT_xS_y T_z acts trivially on . Then S_y T_z must leave the subalgebra (0,0,A) invariant. Hence so must S_y. This forces y = 0, so that g = WT_{x + z}. But then T_x+z must leave the subalgebra (A,0,0) invariant, so that x + z = 0 and g = W. If W acts trivially, W = I.^[21]

The group G can thus be identified with its image in GL .

Let A = E + iE be the complexification of a Euclidean Jordan algebra E. For a = x + iy, set a* = x − iy. The trace form on E defines a complex inner product on A and hence an adjoint operation. The unitary structure group Γ_u(A) consists of those g in Γ(A) that are in U(A), i.e. satisfy gg*=g*g = I. It ia a closed subgroup of U(A). Its Lie algebra consists of the skew-adjoint elements in . Define a conjugate linear involution θ on by

This is a period 2 conjugate-linear automorphism of the Lie algebra. It induces an automorphism of G, which on the generators is given by

Let H be the fixed point subgroup of θ in G. Let be the fixed point subalgebra of θ in . Define a sesquilinear form on by (a,b) = −B(a,θ(b)). This defines a complex inner product on which restricts to a real inner product on . Both are preserved by H. Let K be the identity component of Γ_u(A). It lies in H. Let K_e = T^m be the diagonal torus associated with a Jordan frame in E. The action of SL(2,C)^m is compatible with θ which sends a unimodular matrix to . In particular this gives a homomorphism of SU(2)^m into H.

Now every matrix M in SU(2) can be written as a product

The factor in the middle gives another maximal torus in SU(2) obtained by conjugating by J. If a = ∑ α_ie_i with |α_i| = 1, then Q(a) gives the action of the diagonal torus T = T^m and corresponds to an element of K ⊆ H. The element J lies in SU(2)^m and its image is a Möbius transformation j lying in H. Thus S = j ∘ T ∘ j is another torus in H and T ∘ S ∘ T coincides with the image of SU(2)^m.

Since Z = SU(2)^m(0:0) for the compact complex manifold corresponding to A^e, if follows that Y = T S (0:0), where Y is the image of Z. On the other hand, X = KY, so that X = KTS(0:0) = KS(0:0). On the other hand, the stabilizer of (0:0) in H is K, since the fixed point subgroup of G₀G₋₁ under θ is K. Hence H = KSK. In particular H is compact and connected since both K and S are. Because it is a closed subgroup of U , it is a Lie group. It contains K and hence its Lie algebra contains the operators (0,T,0) with T* = −T. It contains the image of SU(2)^m and hence the elements (a,0,a*) with a in A_e. Since A = KA_e and (k^t)⁻¹(a*) = (ka)*, it follows that the Lie algebra of H contains (a,0,a*) for all a in A. Thus it contains .

They are equal because all skew-adjoint derivations of are inner. In fact, since H normalizes and the action by conjugation is faithful, the map of into the Lie algebra of derivations of is faithful. In particular has trivial center. To show that equals , it suffices to show that coincides with . Derivations on are skew-adjoint for the inner product given by minus the Killing form. Take the invariant inner product on given by −Tr D₁D₂. Since is invariant under so is its orthogonal complement. They are both ideals in , so the Lie bracket between them must vanjsh. But then any derivation in the orthogonal complement would have 0 Lie bracket with , so must be zero. Hence is the Lie algebra of H. (This also follows from a dimension count since dim X = dim H − dim K.)

The formulas above for the action of W and S_y show that the image of G₀G₋₁ is closed in GL . Since H acts transitively on X and the stabilizer of (0:0) in G is G₀G₋₁, it follows that G = HG₀G₋₁. The compactness of H and closedness of G₀G₋₁ implies that G is closed in GL .

G is a closed subgroup of GL so a real Lie group. Since it contains G_i with i = 0 or ±1, its Lie algebra contains . Since is the complexification of , like all its derivations are inner and it has trivial center. Since the Lie algebra of G normalizes and o is the only element centralizing , as in the compact case the Lie algebra of G must be . (This can also be seen by a dimension count since dim X = dim G − dim G₀G₋₁.) Since it is a complex subspace, G is a complex Lie group. It is connected because it is the continuous image of the connected set H × G₀G₋₁. Since is the complexification of , G is the complexification of H.

Noncompact real form

For a in A the spectral norm ||a|| is defined to be max α_i if a = u ∑ α_ie_i with α_i ≥ 0 and u in K. It is independent of choices and defines a norm on A. Let D be the set of a with ||a|| < 1 and let H* be the identity component of the closed subgroup of G carrying D onto itself. It is generated by K, the Möbius transformations in PSU(1,1) and the image of SU(1,1)^m corresponding to a Jordan frame. Let τ be the conjugate-linear period 2 automorphism of defined by

Let be the fixed point algebra of τ. It is the Lie algebra of H*. It induces a period 2 automorphism of G with fixed point subgroup H*. The group H* acts transitively on D. The stabilizer of 0 is K.^[22]

The noncompact real semisimple Lie group H* acts on X with an open orbit D. As with the action of SU(1,1) on the Riemann sphere, it has only finitely many orbits. This orbit structure can be explicitly described when the Jordan algebra A is simple. Let X₀(r,s) be the subset of A consisting of elements a = u ∑ α_ia_i with exactly r of the α_i less than one and exactly s of them greater than one. Thus 0 ≤ r + s ≤ m. These sets are the intersections of the orbits X(r,s) of H* with X₀. The orbits with r + s = m are open. There is a unique compact orbit X(0,0). It is the Shilov boundary S of D consisting of elements e^ix with x in E, the underlying Euclidean Jordan algebra. X(p,q) is in the closure of X(r,s) if and only if p ≤ r and q ≤ s. In particular S is in the closure of every orbit.^[23]

Jordan algebras with involution

The preceding theory describes irreducible Hermitian symmetric spaces of tube type in terms of unital Jordan algebras. In Loos (1977) general Hermitian symmetric spaces are described by a systematic extension of the above theory to Jordan pairs. In the development of Koecher (1969), however, irreducible Hermitian symmetric spaces not of tube type are described in terms of period two automorphisms of simple Euclidean Jordan algebras. In fact any period 2 automorphism defines a Jordan pair: the general results of Loos (1977) on Jordan pairs can be specialized to that setting.

Let τ be a period two automorphism of a simple Euclidean Jordan algebra E with complexification A. There are corresponding decompositions E = E₊ ⊕ E₋ and A = A₊ ⊕ A₋ into ±1 eigenspaces of τ. Let V ≡ A_τ = A₋. τ is assumed to satisfy the additional condition that the trace form on V defines an inner product. For a in V, define Q_τ(a) to be the restriction of Q(a) to V. For a pair (a,b) in V², define B_τ(a,b) and R_τ(a,b) to be the restriction of B(a,b) and R(a,b) to V. Then V is simple if and only if the only subspaces invariant under all the operators Q_τ(a) and R_τ(a,b) are (0) and V.

The conditions for quasi-invertibility in A show that B_τ(a,b) is invertible if and only if B(a,b) is invertible. The quasi-inverse a^b is the same whether computed in A or V. A space of equivalence classes X_τ can be defined on pairs V². It is a closed subspace of X, so compact. It also has the structure of a complex manifold, modelled on V. The structure group Γ(V) can be defined in terms of Q_τ and it has as a subgroup the unitary structure group Γ_u(V) = Γ(V) ∩ U(V) with identity component K_τ. The group K_τ is the identity component of the fixed point subgroup of τ in K. Let G_τ be the group of biholomorphisms of X_τ generated by W in G_τ,0, the identity component of Γ(V), and the Abelian groups G_τ,−1 consisting of the S_a and G_τ,+1 consisting of the T_b with a and b in V. It acts transitively on X_τ with stabilizer G_τ,0G_τ,−1 and G_τ = G_τ,0G_τ,−1G_τ,+1G_τ,−1. The Lie algebra of holomorphic vector fields on X_τ is a 3-graded Lie algebra,

Restricted to V the components are generated as before by the constant functions into V, by the operators R_τ(a,b) and by the operators Q_τ(a). The Lie brackets are given by exactly the same formula as before.

The spectral decomposition in E_τ and V is accomplished using tripotents, i.e. elements e such that e³ = e. In this case f = e² is an idempotent in E₊. There is a Pierce decomposition E = E₀(f) ⊕ E_½(f) ⊕ E₁(f) into eigenspaces of L(f). The operators L(e) and L(f) commute, so L(e) leaves the eigenspaces above invariant. In fact L(e)² acts as 0 on E₀(f), as 1/4 on E_½(f) and 1 on E₁(f). This induces a Pierce decomposition E_τ = E_τ,0(f) ⊕ E_τ,½(f) ⊕ E_τ,1(f). The subspace E_τ,1(f) becomes a Euclidean Jordan algebra with unit f under the mutation Jordan product x ∘ y = {x,e,y}.

Two tripotents e₁ and e₂ are said to be orthogonal if all the operators [L(a),L(b)] = 0 when a and b are powers of e₁ and e₂ and if the corresponding idempotents f₁ and f₂ are orthogonal. In this case e₁ and e₂ generate a commutative associative algebra and e₁e₂ = 0, since (e₁e₂,e₁e₂) =(f₁,f₂) =0. Let a be in E_τ. Let F be the finite-dimensional real subspace spanned by odd powers of a. The commuting self-adjoint operators L(x)L(y) with x, y odd powers of a act on F, so can be simultaneously diagonalized by an orthonormal basis e_i. Since (e_i)³ is a positive multiple of e_i, rescaling if necessary, e_i can be chosen to be a tripotent. They form an orthogonal family by construction. Since a is in F, it can be written a = ∑ α_i e_i with α_i real. These are called the eigenvalues of a (with respect to τ). Any other tripotent e in F has the form a = ∑ ε_i e_i with ε_i = 0, ±1, so the e_i are up to sign the minimal tripotents in F.

A maximal family of orthogonal tripotents in E_τ is called a Jordan frame. The tripotents are necessarily minimal. All Jordan frames have the same number of elements, called the rank of E_τ. Any two frames are related by an element in the subgroup of the structure group of E_τ preserving the trace form. For a given Jordan frame (e_i), any element a in V can be written in the form a = u ∑ α_i e_i with α_i ≥ 0 and u an operator in K_τ. The spectral norm of a is defined by ||a|| = sup α_i and is independent of choices. Its square equals the operator norm of Q_τ(a). Thus V becomes a complex normed space with open unit ball D_τ.

Note that for x in E, the operator Q(x) is self-adjoint so that the norm ||Q(x)ⁿ|| = ||Q(x)||ⁿ. Since Q(x)ⁿ = Q(xⁿ), it follows that ||xⁿ|| = ||x||ⁿ. In particular the spectral norm of x = ∑ α_i e_i in A is the square root of the spectral norm of x² = ∑ (α_i)² f_i. It follows that the spectral norm of x is the same whether calculated in A or A_τ. Since K_τ preserves both norms, the spectral norm on A_τ is obtained by restricting the spectral norm on A.

For a Jordan frame e₁, ..., e_m, let V_e = ⊕ C e_i. There is an action of SL(2,C)^m on V_e which extends to V. If c = ∑ γ_ie_i and b = ∑ β_ie_i, then S(c) and T(b) give the action of the product of the lower and upper unitriangular matrices. If a = ∑ α_ie_i with α_i ≠ 0, then the corresponding product of diagonal matrices act as W = B_τ(a, e − a), where e = ∑ e_i.^[24] In particular the diagonal matrices give an action of (C*)^m and T^m.

As in the case without an automorphism τ, there is an automorphism θ of G_τ. The same arguments show that the fixed point subgroup H_τ is generated by K_τ and the image of SU(2)^m. It is a compact connected Lie group. It acts transitively on X_τ; the stabilizer of (0:0) is K_τ. Thus X_τ = H_τ/K_τ, a Hermitian symmetric space of compact type.

Let H_τ* be the identity component of the closed subgroup of G_τ carrying D_τ onto itself. It is generated by K_τ and the image of SU(1,1)^m corresponding to a Jordan frame. Let ρ be the conjugate-linear period 2 automorphism of defined by

Let be the fixed point algebra of ρ. It is the Lie algebra of H_τ*. It induces a period 2 automorphism of G with fixed point subgroup H_τ*. The group H_τ* acts transitively on D_τ. The stabilizer of 0 is K_τ*.^[25] H_τ*/K_τ is the Hermitian symmetric space of noncompact type dual to H_τ/K_τ.

The Hermitian symmetric space of non-compact type have an unbounded realization, analogous the upper half-plane in C. Möbius transformations in PSL(2,C) corresponding to the Cayley transform and its inverse give biholomorphisms of the Riemann sphere exchanging the unit disk and the upper halfplane. When the Hermitian symmetric space is of tube type the same Möbius transformations map the disk D in A onto the tube domain T = E + iC were C is the open self-dual convex cone of squares in the Euclidean Jordan algebra E.

For Hermitian symmetric space not of tube type there is no action of PSL(2,C) on X, so no analogous Cayley transform. A partial Cayley transform can be defined in that case for any given maximal tripotent e = ∑ ε_i e_i in E_τ. It takes the disk D_τ in A_τ = A_τ,1(f) ⊕ A_τ,½(f) onto a Siegel domain of the second kind.

In this case E_τ,1(f) is a Euclidean Jordan algebra and there is symmetric E_τ,1(f)-valued bilinear form on E_τ,½(f) such that the corresponding quadratic form q takes values in its positive cone C_τ. The Siegel domain consists of pairs (x + iy,u + iv) such that y − q(u) − q(v) lies in C_τ. The quadratic form q on E_τ,½(f) and the squaring operation on E_τ,1(f) are given by x ↦ Q_τ(x)e. The positive cone C_τ corresponds to x with Q_τ(x) invertible.^[26]

Examples

For simple Euclidean Jordan algebras E with complexication A, the Hermitian symmetric spaces of compact type X can be described explicitly as follows, using Cartan's classification.^[27]

Type I_n. A is the Jordan algebra of n × n complex matrices M_n(C) with the operator Jordan product x ∘ y = ½(xy + yx). It is the complexification of E = H_n(C), the Euclidean Jordan algebra of self-adjoint n × n complex matrices. In this case G = PSL(2n,C) acting on A with acting as g(z) = (az + b)(cz + d)⁻¹. Indeed, this can be verified directly for diagonal, upper and lower unitriangular matrices which correspond to the operators W, S_c and T_b. The subset Ω corresponds to the matrices g with d invertible. In fact consider the space of linear maps from Cⁿ to C²ⁿ = Cⁿ ⊕ Cⁿ. It is described by a pair (T₁|T₂) with T_i in M_n(C). This is a module for GL(2n,C) acting on the target space. There is also an action of GL(n,C) induced by the action on the source space. The space of injective maps U is invariant and GL(n,C) acts freely on it. The quotient is the Grassmannian M consisting of n-dimensional subspaces of C²ⁿ. Define a map of A² into M by sending (a,b) to the injective map (a|I − b^ta). This map induces an isomorphism of X onto M.

In fact let V be an n-dimensional subspace of Cⁿ ⊕ Cⁿ. If it is in general position, i.e. it and its orthogonal complement have trivial intersection with Cⁿ ⊕ (0) and (0) ⊕ Cⁿ, it is the graph of an invertible operator T. So the image corresponds to (a|I − b^ta) with a = I and b^t = I − T.

At the other extreme, V and its orthogonal complement U can be written as orthogonal sums V = V₁ ⊕ V₂, U = U₁ ⊕ U₂, where V₁ and U₁ are the intersections with Cⁿ ⊕ (0) and V₂ and U₂ with (0) ⊕ Cⁿ. Then dim V₁ = dim U₂ and dim V₂ = dim U₁. Moreover, Cⁿ ⊕ (0) = V₁ ⊕ U₁ and (0) ⊕ Cⁿ = V₂ ⊕ U₂. The subspace V corresponds to the pair (e|I − e), where e is the orthogonal projection of Cⁿ ⊕ (0) onto V₁. So a = e and b = I.

The general case is a direct sum of these two cases. V can be written as an orthogonal sum V = V₀ ⊕ V₁ ⊕ V₂ where V₁ and V₂ are the intersections with Cⁿ ⊕ (0) and (0) ⊕ Cⁿ and V₀ is their orthogonal complement in V. Similarly the orthogonal complement U of V can be written U = U₀ ⊕ U₁ ⊕ U₂. Thus Cⁿ ⊕ (0) = V₁ ⊕ U₁ ⊕ W₁ and (0) ⊕ Cⁿ = V₂ ⊕ U₂ ⊕ W₂, where W_i are orthogonal complements. The direct sum (V₁ ⊕ U₁) ⊕ (V₂ ⊕ U₂) ⊆ Cⁿ ⊕ Cⁿ is of the second kind and its orthogonal complement of the first.

Maps W in the structure group correspond to h in GL(n,C), with W(a) = hah^t. The corresponding map on M sends (x|y) to (hx|(h^t)⁻¹y). Similarly the map corresponding to S_c sends (x|y) to (x|y + c), the map corresponding to T_b sends (x|y) to (x + b|y) and the map corresponding to J sends (x|y) to (y|−x). It follows that the map corresponding to g sends (x|y) to (ax + by|cx + dy). On the other hand, if y is invertible, (x|y) is equivalent to (xy⁻¹|I), whence the formula for the fractional linear transformation.

Type III_n. A is the Jordan algebra of n × n symmetric complex matrices S_n(C) with the operator Jordan product x ∘ y = ½(xy + yx). It is the complexification of E = H_n(R), the Euclidean Jordan algebra of n × n symmetric real matrices. On C²ⁿ = Cⁿ ⊕ Cⁿ, define a nondegenerate alternating bilinear form by ω(x₁ ⊕ y₁, x₂ ⊕ y₂) = x₁ • y₂ − y₁ • x₂. In matrix notation if ,

Let Sp(2n,C) denote the complex symplectic group, the subgroup of GL(2n,C) preserving ω. It consists of g such that gJg^t = J and is closed under g ↦ g^t. If belongs to Sp(2n,C) then

It has center {±I}. In this case G = Sp(2n,C)/{±I} acting on A as g(z) = (az + b)(cz + d)⁻¹. Indeed, this can be verified directly for diagonal, upper and lower unitriangular matrices which correspond to the operators W, S_c and T_b. The subset Ω corresponds to the matrices g with d invertible. In fact consider the space of linear maps from Cⁿ to C²ⁿ = Cⁿ ⊕ Cⁿ. It is described by a pair (T₁|T₂) with T_i in M_n(C). This is a module for Sp(2n,C) acting on the target space. There is also an action of GL(n,C) induced by the action on the source space. The space of injective maps U with isotropic image, i.e. ω vanishes on the image, is invariant. Moreover, GL(n,C) acts freely on it. The quotient is the symplectic Grassmannian M consisting of n-dimensional Lagrangian subspaces of C²ⁿ. Define a map of A² into M by sending (a,b) to the injective map (a|I − ba). This map induces an isomorphism of X onto M.

In fact let V be an n-dimensional Lagrangian subspace of Cⁿ ⊕ Cⁿ. Let U be a Lagrangian subspace complementing V. If they are in general position, i.e. they have trivial intersection with Cⁿ ⊕ (0) and (0) ⊕ Cⁿ, than V is the graph of an invertible operator T with T^t = T. So the image corresponds to (a|I − ba) with a = I and b = I − T.

At the other extreme, V and U can be written as direct sums V = V₁ ⊕ V₂, U = U₁ ⊕ U₂, where V₁ and U₁ are the intersections with Cⁿ ⊕ (0) and V₂ and U₂ with (0) ⊕ Cⁿ. Then dim V₁ = dim U₂ and dim V₂ = dim U₁. Moreover, Cⁿ ⊕ (0) = V₁ ⊕ U₁ and (0) ⊕ Cⁿ = V₂ ⊕ U₂. The subspace V corresponds to the pair (e|I − e), where e is the projection of Cⁿ ⊕ (0) onto V₁. Note that the pair (Cⁿ ⊕ (0), (0) ⊕ Cⁿ) is in duality with respect to ω and the identification between them induces the canonical symmetric bilinear form on Cⁿ. In particular V₁ is identified with U₂ and V₂ with U₁. Moreover, they are V₁ and U₁ are orthogonal with respect to the symmetric bilinear form on (Cⁿ ⊕ (0). Hence the idempotent e satisfies e^t = e. So a = e and b = I lie in A and V is the image of (a|I − ba).

The general case is a direct sum of these two cases. V can be written as a direct sum V = V₀ ⊕ V₁ ⊕ V₂ where V₁ and V₂ are the intersections with Cⁿ ⊕ (0) and (0) ⊕ Cⁿ and V₀ is a complement in V. Similarly U can be written U = U₀ ⊕ U₁ ⊕ U₂. Thus Cⁿ ⊕ (0) = V₁ ⊕ U₁ ⊕ W₁ and (0) ⊕ Cⁿ = V₂ ⊕ U₂ ⊕ W₂, where W_i are complements. The direct sum (V₁ ⊕ U₁) ⊕ (V₂ ⊕ U₂) ⊆ Cⁿ ⊕ Cⁿ is of the second kind. It has a complement of the first kind.

Maps W in the structure group correspond to h in GL(n,C), with W(a) = hah^t. The corresponding map on M sends (x|y) to (hx|(h^t)⁻¹y). Similarly the map corresponding to S_c sends (x|y) to (x|y + c), the map corresponding to T_b sends (x|y) to (x + b|y) and the map corresponding to J sends (x|y) to (y|−x). It follows that the map corresponding to g sends (x|y) to (ax + by|cx + dy). On the other hand, if y is invertible, (x|y) is equivalent to (xy⁻¹|I), whence the formula for the fractional linear transformation.

Type II_2n. A is the Jordan algebra of 2n × 2n skew-symmetric complex matrices A_n(C) and Jordan product x ∘ y = −½(x J y + y J x) where the unit is given by . It is the complexification of E = H_n(H), the Euclidean Jordan algebra of self-adjoint n × n matrices with entries in the quaternions. This is discussed in Loos (1977) and Koecher (1969).

Type IV_n. A is the Jordan algebra Cⁿ ⊕ C with Jordan product (x,α) ∘ (y,β) = (βx + αy,αβ + x•y). It is the complexication of the rank 2 Euclidean Jordan algebra defined by the same formulas but with real coefficients. This is discussed in Loos (1977).

Type VI. The complexified Albert algebra. This is discussed in Faulkner (1972), Loos (1978) and Drucker (1981).

The Hermitian symmetric spaces of compact type X for simple Euclidean Jordan algebras E with period two automorphism can be described explicitly as follows, using Cartan's classification.^[28]

Type I_p,q. Let F be the space of q by p matrices over R with p ≠ q. This corresponds to the automorphism of E = H_{p + q}(R) given by conjugating by the diagonal matrix with p diagonal entries equal to 1 and q to −1. Without loss of generality p can be taken greater than q. The structure is given by the triple product xy^tz. The space X can be identified with the Grassmannian of p-dimensional subspace of C^{p + q} = C^p ⊕ C^q. This has a natural embedding in C^2p = C^p ⊕ C^p by adding 0's in the last p − q coordinates. Since any p-dimensional subspace of C^2p can be represented in the form [I − y^tx|x], the same is true for subspaces lying in C^{p + q}. The last p − q rows of x must vanish and the mapping does not change if the last p − q rows of y are set equal to zero. So a similar representation holds for mappings, but now with q by p matrices. Exactly as when p = q, it follows that there is an action of GL(p + q, C) by fractional linear transformations.^[29]

Type II_n F is the space of real skew-symmetric m by m matrices. After removing a factor of √(-1), this corresponds to the period 2 automorphism given by complex conjugation on E = H_n(C).

Type V. F is the direct sum of two copies of the Cayley numbers, regarded as 1 by 2 matrices. This corresponds to the canonical period 2 automorphism defined by any minimal idempotent in E = H₃(O).

See also

• Mutation (algebra)
• Symmetric cone

Notes

References

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