In mathematics, the determinant of a skew-symmetric matrix can always be written as the square of a polynomial in the matrix entries. This polynomial is called the Pfaffian of the matrix, The term Pfaffian was introduced by Cayley (1852) who named them after Johann Friedrich Pfaff. The Pfaffian is nonvanishing only for 2n × 2n skew-symmetric matrices, in which case it is a polynomial of degree n.
Explicitly, for a skew-symmetric matrix A,
which was first proved by Thomas Muir in 1882 (Muir 1882).
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(3 x 3 is odd, so Pfaffian of B is 0)
The Pfaffian of a 2n × 2n skew-symmetric tridiagonal matrix is given as
which contains the important case of a 2n × 2n skew-symmetric matrix with 2 × 2 blocks on the diagonal:
(Note that any skew-symmetric matrix can be reduced to this form, see Spectral theory of a skew-symmetric matrix)
Let A = {ai,j} be a 2n × 2n skew-symmetric matrix. The Pfaffian of A is defined by the equation
where S2n is the symmetric group and sgn(σ) is the signature of σ.
One can make use of the skew-symmetry of A to avoid summing over all possible permutations. Let Π be the set of all partitions of {1, 2, …, 2n} into pairs without regard to order. There are (2n − 1)!! such partitions. An element α ∈ Π can be written as
with ik < jk and . Let
be the corresponding permutation. Given a partition α as above, define
The Pfaffian of A is then given by
The Pfaffian of a n×n skew-symmetric matrix for n odd is defined to be zero, as the determinant of an odd skew-symmetric matrix is zero, since for a skew-symmetric matrix, , and for n odd, this implies .
By convention, the Pfaffian of the 0×0 matrix is equal to one. The Pfaffian of a skew-symmetric 2n×2n matrix A with n>0 can be computed recursively as
where denotes the matrix A with both the first and i-th rows and columns removed.
where {e1, e2, …, e2n} is the standard basis of R2n. The Pfaffian is then defined by the equation
here ωn denotes the wedge product of n copies of ω with itself.
For a 2n × 2n skew-symmetric matrix A
For an arbitrary 2n × 2n matrix B,
For a block-diagonal matrix
For an arbitrary n × n matrix M:
If A depends on some variable xi, then the gradient of Pfaffian is given by
and the Hessian of Pfaffian is given by
Pfaffians have the following properties similar to that of determinants
These properties can be derived from the identity .