topology (point-set topology, point-free topology)
see also differential topology, algebraic topology, functional analysis and topological homotopy theory
Basic concepts
fiber space, space attachment
Extra stuff, structure, properties
Kolmogorov space, Hausdorff space, regular space, normal space
sequentially compact, countably compact, locally compact, sigma-compact, paracompact, countably paracompact, strongly compact
Examples
Basic statements
closed subspaces of compact Hausdorff spaces are equivalently compact subspaces
open subspaces of compact Hausdorff spaces are locally compact
compact spaces equivalently have converging subnet of every net
continuous metric space valued function on compact metric space is uniformly continuous
paracompact Hausdorff spaces equivalently admit subordinate partitions of unity
injective proper maps to locally compact spaces are equivalently the closed embeddings
locally compact and second-countable spaces are sigma-compact
Theorems
Analysis Theorems
manifolds and cobordisms
cobordism theory, Introduction
This is one of the parallelizable spheres, as such corresponds to the octonions among the division algebras, being the manifold of unit octonions, and is the only one of these which does not carry (Lie) group structure but just Moufang loop structure.
The 7-sphere participates in the quaternionic Hopf fibration, the analog of the complex Hopf fibration with the field of complex numbers replaced by the division ring of quaternions or Hamiltonian numbers $\mathbb{H}$.
Here the idea is that $S^7$ can be construed as $\{(x, y) \in \mathbb{H}^2: {|x|}^2 + {|y|}^2 = 1\}$, with $p$ mapping $(x, y)$ to $x/y$ as an element in the projective line $\mathbb{P}^1(\mathbb{H}) \cong S^4$, with each fiber a torsor parametrized by quaternionic scalars $\lambda$ of unit norm (so $\lambda \in S^3$). This canonical $S^3$-bundle (or $SU(2)$-bundle) is classified by a map $S^4 \to \mathbf{B} SU(2)$.
A celebrated result of Milnor is that $S^7$ admits exotic smooth structures (exotic spheres), i.e., there are smooth manifold structures on the topological manifold $S^7$ that are not diffeomorphic to the standard smooth structure on $S^7$. More structurally, considering smooth structures up to oriented diffeomorphism, the different smooth structures form a monoid under a (suitable) operation of connected sum, and this monoid is isomorphic to the cyclic group $\mathbb{Z}/(28)$. With the notable possible exception of $n = 4$ (where the question of existence of exotic 4-spheres is wide open), exotic spheres first occur in dimension $7$. This phenomenon is connected to the h-cobordism theorem (the monoid of smooth structures is identified with the monoid of h-cobordism classes of oriented homotopy spheres?).
One explicit construction of the smooth structures is given as follows (see Milnor 1968). Let $W_k$ be the algebraic variety in $\mathbb{C}^5$ defined by the equation
and $S_\epsilon \subset \mathbb{C}^5$ a sphere of small radius $\epsilon$ centered at the origin. Then each of the $28$ smooth structures on $S^7$ is represented by an intersection $W_k \cap S_\epsilon$, as $k$ ranges from $1$ to $28$. These manifolds sometimes go by the name Brieskorn manifolds or Brieskorn sphere?s^{1}.
Let $\phi_0 \in \Omega^3(\mathbb{R}^7)$ be the associative 3-form and let
be given by
(where $x_0$ denotes the canonical coordinate on the first factor of $\mathbb{R}$ and $\phi_0$ is pulled back along the projection to $\mathbb{R}^7$) .
By construction this is its own Hodge dual
This implies that as we restrict $\Phi_0$ to
then there is a unique 3-form
on the 7-sphere such that
This 3-form $\phi$ defines a G2-structure on $S^7$. It is nearly parallel in that
(e.g. Lotay 12, def.2.4)
Martin Cederwall, Christian R. Preitschopf, The Seven-sphere and its Kac-Moody Algebra, Commun. Math. Phys. 167 (1995) 373-394 (arXiv:hep-th/9309030)
Takeshi Ôno, On the Hopf fibration $S^7 \to S^4$ over $Z$, Nagoya Math. J. Volume 59 (1975), 59-64. (Euclid)
Relation to the Milnor fibration:
An ADE classification of finite subgroups of $SO(8)$ acting freely on $S^7$ (see at group action on an n-sphere) such that the quotient is spin and has at least four Killing spinors (see also at ABJM model) is in
Paul de Medeiros, José Figueroa-O'Farrill, Sunil Gadhia, Elena Méndez-Escobar, Half-BPS quotients in M-theory: ADE with a twist, JHEP 0910:038,2009 (arXiv:0909.0163, pdf slides)
Paul de Medeiros, José Figueroa-O'Farrill, Half-BPS M2-brane orbifolds (arXiv:1007.4761)
Discussion of exotic smooth structures on 7-spheres includes
The explicit construction of exotic 7-spheres by intersecting algebraic varieties with spheres is described in
Discussion of (nearly) G2-structures on $S^7$ and calibrated submanifolds includes
Not surprisingly, these exotic spheres are also called Milnor spheres. ↩
Last revised on July 28, 2018 at 09:23:03. See the history of this page for a list of all contributions to it.