Contents

# Contents

## Idea

The quaternionic projective space $\mathbb{H}P^n$ is the space of right (or left) quaternion lines through the origin in $\mathbb{H}^{n+1}$, hence the space of equivalence classes $[q_1, \cdots, q_{n+1}]$ of (n+1)-tuples of quaternions excluding zero, under the equivalence relation given by right (or left) multiplication with non-zero quaternions

$\mathbb{H}P^n \;\coloneqq\; \big\{ [q_1, \cdots, q_{n+1}] \big\} \;\coloneqq\; \Big( \big\{ (q_1, \cdots, q_{n+1}) \big\} \setminus \{(0, \cdots, 0)\} \Big) /_{ (q_1, \cdots, q_{n+1}) \sim (q_1 q, \cdots, q_{n+1} q) \vert q \neq 0 }$

## Properties

### As a coset space

As any Grassmannian, quaternion projective space is canonically a coset space, in this case of the quaternion unitary group $Sp(n+1)$ by the central product group Sp(n).Sp(1):

(1)$\mathbb{H}P^n \;\simeq\; \frac{ Sp(n+1) }{ Sp(n)\cdot Sp(1) }$

### As a quaternion-Kähler symmetric space (Wolf space)

By the coset space-realization (1), quaternion projective space is naturally a quaternion-Kähler manifold which is also a symmetric space. As such it is an example of a Wolf space.

### General

M-theory on the 8-manifold$\;$ HP2, hence on a quaternion-Kähler manifold of dimension 8 with holonomy Sp(2).Sp(1), is considered in
• Michael Atiyah, Edward Witten, p. 75 onwards in $M$-Theory dynamics on a manifold of $G_2$-holonomy, Adv. Theor. Math. Phys. 6 (2001) (arXiv:hep-th/0107177)