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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 Quaternionic projective space of dimension *n* is usually denoted by

and is a closed manifold of (real) dimension 4*n*. It is a homogeneous space for a Lie group action, in more than one way. The quaternionic projective line is homeomorphic to the 4-sphere.

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

where the 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

- .

In the language of group actions, is the orbit space of by the action of , the multiplicative group of non-zero quaternions. By first projecting onto the unit sphere inside one may also regard as the orbit space of by the action of , the group of unit quaternions.^{[1]} The sphere then becomes a principal Sp(1)-bundle over :

This bundle is sometimes called a (generalized) Hopf fibration.

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

The space , defined as the union of all finite 's under inclusion, is the classifying space *BS*^{3}. The homotopy groups of are given by These groups are known to be very complex and in particular they are non-zero for infinitely many values of . However, we do have that

It follows that rationally, i.e. after localisation of a space, is an Eilenberg–Maclane space . That is (cf. the example K(Z,2)). See rational homotopy theory.

In general, has a cell structure with one cell in each dimension which is a multiple of 4, up to . Accordingly, its cohomology ring is , where is a 4-dimensional generator. This is analogous to complex projective space. It also follows from rational homotopy theory that has infinite homotopy groups only in dimensions 4 and .

carries a natural Riemannian metric analogous to the Fubini-Study metric on , with respect to which it is a compact quaternion-Kähler symmetric space with positive curvature.

Quaternionic projective space can be represented as the coset space

where is the compact symplectic group.

Since , its tangent bundle is stably trivial. The tangent bundles of the rest have nontrivial Stiefel–Whitney and Pontryagin classes. The total classes are given by the following formulas:

where is the generator of and is its reduction mod 2.^{[2]}

The one-dimensional projective space over is called the "projective line" in generalization of the complex projective line. For example, it was used (implicitly) in 1947 by P. G. Gormley to extend the Möbius group to the quaternion context with linear fractional transformations. For the linear fractional transformations of an associative ring with 1, see projective line over a ring and the homography group GL(2,*A*).

From the topological point of view the quaternionic projective line is the 4-sphere, and in fact these are diffeomorphic manifolds. The fibration mentioned previously is from the 7-sphere, and is an example of a Hopf fibration.

Explicit expressions for coordinates for the 4-sphere can be found in the article on the Fubini–Study metric.

The 8-dimensional 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

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.

**^**Naber, Gregory L. (2011) [1997]. "Physical and Geometrical Motivation".*Topology, Geometry and Gauge fields*. Texts in Applied Mathematics. Vol. 25. Springer. p. 50. doi:10.1007/978-1-4419-7254-5_0. ISBN 978-1-4419-7254-5.**^**Szczarba, R.H. (1964). "On tangent bundles of fibre spaces and quotient spaces" (PDF).*American Journal of Mathematics*.**86**(4): 685–697. doi:10.2307/2373152. JSTOR 2373152.

- Arnol'd, V.I. (1999). "Relatives of the Quotient of the Complex Projective Plane by the Complex Conjugation".
*Tr. Mat. Inst. Steklova*.**224**: 56–6. CiteSeerX 10.1.1.50.6421. Treats the analogue of the result mentioned for quaternionic projective space and the 13-sphere. - Gormley, P.G. (1947), "Stereographic projection and the linear fractional group of transformations of quaternions",
*Proceedings of the Royal Irish Academy, Section A*,**51**: 67–85, JSTOR 20488472