geometric representation theory
representation, 2-representation, ∞-representation
Grothendieck group, lambda-ring, symmetric function, formal group
principal bundle, torsor, vector bundle, Atiyah Lie algebroid
Eilenberg-Moore category, algebra over an operad, actegory, crossed module
Be?linson-Bernstein localization?
higher geometry / derived geometry
Ingredients
Concepts
geometric little (∞,1)-toposes
geometric big (∞,1)-toposes
Constructions
Examples
derived smooth geometry
Theorems
In the context of real-oriented cohomology theory, notably in KR-theory, by “real space” (“real manifold”) one means a topological space (a manifold) equipped with an action of the cyclic group of order 2 $\mathbb{Z}/2$ (a “non-linear real structure”).
In the context of string theory real spaces appear as orientifold target spacetimes. The involution fixed points here are known as O-planes.
There are three non-equivalent real structures on the circle $S^1$, usually denoted
$S^{2,0}$ for the trivial involution;
$S^{1,1}$ for the reflection involution (identifying the two semi-circles);
$S^{0,2}$ for the antipodal involution (rotation by $\pi$).
Accordingly real-oriented cohomology theory is bigraded in a way modeled on this bigrading.
(This is standard notation, but maybe $S^{1,0}$, $S^{\tfrac{1}{2}, \tfrac{1}{2}}, S^{0,1}$ would be more suggestive. Indeed the quotients in the first and the last case are actually circles, while in the second case it is the semi-circle.)
The complex $n$-dimensional complexified cartesian space ${\mathbb{C}}^n$ equipped with its conjugative involution is a real space. Explicitly, this involution sends $(z^1,\ldots, z^n)$ to $(\bar{z^1},\ldots, \bar{z^n})$.
The complex $n$-dimensional complex projective space ${\mathbb{P}}^n_{\mathbb{C}}$ equipped with a conjugation involution is a real space. For each choice of affine chart, the conjugation involution of this chart (which is biholomorphic to ${\mathbb{C}}^n$) extends to a conjugation involution on ${\mathbb{P}}^n_{\mathbb{C}}$. Any two conjugation involutions are ${\mathbb{Z}}/2$-equivariantly diffeomorphic.
For example, the Riemann surface ${\mathbb{P}}^1_{\mathbb{C}}$ is diffeomorphic to the 2-sphere $S^2$ and its conjugation involution is the antipodal action.
For $A$ a square matrix, the determinant of its conjugation transpose equals the conjugate of its determinant. In symbols, ${\mathrm{det}} A^* = \overline{{\mathrm{det}} A}$. Hence sending a square matrix to its conjugate transpose is an involution on the complex general $GL(n,{\mathbb{C}})$ and special $SL(n,{\mathbb{C}})$.
In particular, the real space $SL(1,{\mathbb{C}})$ equipped with this conjugate transpose involution is equivariantly diffeomorphic to $S^{0,2}$, the circle equipped with its antipodal action (dicussed above).
There is a “real” analog of complex cobordism cohomology theory $MU$, the MR cohomology theory $M \mathbb{R}$.
While $\pi_\bullet(M \mathbb{R})$ is not the cobordism ring of real manifolds, still every real manifold does give a class in $M \mathbb{R}$ (Kriz 01, p. 13). For details see here: pdf.
Michael Atiyah, K-theory and reality, The Quarterly Journal of Mathematics. Oxford. Second Series 17 1 (1966) 367-386 [doi:10.1093/qmath/17.1.367, pdf, ISSN:0033-5606]
Igor Kriz, Real-oriented homotopy theory and an analogue of the Adams-Novikov spectral sequence, Topology 40 (2001) 317-399 (pdf)
Last revised on September 6, 2023 at 09:23:26. See the history of this page for a list of all contributions to it.