Quaternionic projective space

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In mathematics, quaternionic projective space is an extension of the ideas of real projective space and complex projective space, to the case where coordinates lie in the ring of quaternions H. Quaternionic projective space of dimension n is usually denoted by

HPⁿ

and is a closed manifold of (real) dimension 4n. It is a homogeneous space for a Lie group action, in more than one way.

Its direct construction is as a special case of the projective space over a division algebra. The homogeneous coordinates of a point can be written

[q₀:q₁: ... :q_n]

where the q_i are quaternions, not all zero. Two sets of coordinates represent the same point if they are 'proportional' by a left multiplication by a non-zero quaternion c; that is, we identify all the

[cq₀:cq₁: ... :cq_n].

In the language of group actions, HPⁿ is the orbit space of Hⁿ⁺¹ by the action of H*, the multiplicative group of non-zero quaternions. By first projecting onto the unit sphere inside Hⁿ⁺¹ one may also regard HPⁿ as the orbit space of S⁴ⁿ⁺³ by the action of Sp(1), the group of unit quaternions. The sphere S⁴ⁿ⁺³ then becomes a principal Sp(1)-bundle over HPⁿ:

There is also a construction of HPⁿ by means of two-dimensional complex subspaces of C²ⁿ, meaning that HPⁿ lies inside a complex Grassmannian.

[edit] Quaternionic projective plane

The 8-dimensional HP² has a circle action, by the group of complex scalars of absolute value 1 acting on the other side (so on the right, as the convention for the action of c above is on the left). Therefore the quotient manifold

HPⁿ/U(1)

may be taken, writing U(1) for the circle group. It has been shown that this quotient is the 7-sphere, a result of Vladimir Arnold from 1996, later rediscovered by Edward Witten and Michael Atiyah.