geometry, complex numbers, complex line
$dim = 1$: Riemann surface, super Riemann surface
homotopy theory, (∞,1)-category theory, homotopy type theory
flavors: stable, equivariant, rational, p-adic, proper, geometric, cohesive, directed…
models: topological, simplicial, localic, …
see also algebraic topology
Introductions
Definitions
Paths and cylinders
Homotopy groups
Basic facts
Theorems
On the stable homotopy type of mapping spaces of (regular) rational maps.
Under good conditions, the subspace inclusion of the space of rational maps (regular, see Rem. ), between given projective complex manifolds or algebraic varieties, into the mapping space of all continuous maps
induces an isomorphism in integral ordinary homology in low degrees, while for maps out of the Riemann sphere this is even an isomorphism on homotopy groups in low degrees:
This phenomenon originates in results of Segal 1979 and is commonly referred to by Segal’s name (e.g. “theorems of Segal-type” in Friedlander & Lawson 1997, Sec. 5.C.
Whenever this holds it provides
from left to right: homotopy theoretic tools for analyzing moduli spaces of rational hypersurfaces;
from right to left: small algebraic models for stable homotopy types of mapping spaces
at least up to some dimension.
Some remarks on the terminology being used:
(“degree”)
Most or all of the following statement invoke an integer “degree” of continuous functions. Beware that this is not the degree of a continuous function (see there) in the usual sense of algebraic topology, except in special cases (such as the archetypical example ).
(“rational maps”)
It it tradition (starting with Segal) to speak of rational maps in the following, but in the end the focus on regular rational maps (“morphisms”: e.g. Friedlander & Lawson 1997, p. 27)), as is necessary to regard them as continuous functions defined everywhere on the given domain $X_1$.
In many cases of interest, such as when the domain $X_1$ is a non-singular complex curve/Riemann surface and the codomain $X_2$ a complex projective space, then all rational maps from $X_1$ to $X_2$ are automatically regular (e.g. Shafarevich Vol1, Cor. 2.3).
For $n \in \mathbb{N}_+$, consider complex projective n-space $\mathbb{C}P^n$.
Say that a continuous map $f \;\colon\; \Sigma_2 \to \mathbb{C}P^n$ out of a 2-dimensional manifold has degree $d \in \mathbb{N}$ (Rem. ) if the pullback of the generator $1 \in \mathbb{Z} \simeq H^2\big( \mathbb{C}P^n;\, \mathbb{Z}\big)$ (see here) is
For $\Sigma$ a compact connected Riemann surface write $g_\Sigma \in \mathbb{N}$ for its genus.
(Segal’s theorem)
For $\Sigma$ a compact connected Riemann surface, the inclusion
of
the topological subspace of rational maps to complex projective n-space
(the moduli spaces of rational complex curves in $\mathbb{C}P^n$)
into
is
for $g_\Sigma = 0$ (i.e. $\Sigma$ the Riemann sphere):
an isomorphism on homotopy groups in degrees $\leq d (2 n -1)$;
for $g_\Sigma \geq 1$:
an isomorphism on ordinary homology groups in degrees $\leq (d - 2g) (2 n -1)$.
(the archetypical case)
In the special case that $n = 1$ and $\Sigma = S^2$ is the 2-sphere with its complex structure, so that both domain and codomain are the Riemann sphere $\mathbb{C}P^1$, Prop. says that
is an isomorphism on homotopy groups up to degree $\leq d$.
(relation to Yang-Mills monopoles)
Example controls the classification of Yang-Mills monopoles. See there for more
(relation to Gromov-Witten theory) A compactification and quotient stack of the space of rational maps in (2) is considered in Gromov-Witten theory, e.g. Bertram 2002, p. 9.
(relation to twistor string theory)
In the context of twistor string theory, the spaces of rational maps $\Sigma \to \mathbb{C}P^3$ (2) are interpreted as moduli spaces of D1-brane-instantons in the twistor space $\mathbb{C}P^3$ (Witten 2004, Sec. 3).
Such rational maps are also argued to encode scattering amplitudes in D=4 N=8 supergravity (Cachazo & Skinner 2012, Adamo 2015).
Here the number of poles in the rational function is the number $n$ of particles in the n-point function, and the genus and degree encode the particle’s helicity and the loop order of the scattering amplitude.
(comparison to the homotopical Oka principle)
Prop. may be compared to the homotopical Oka principle, which applies (since $\mathbb{C}P^n$ is an Oka manifold by this Prop.) to the complementary case of connected non-compact (“open”) Riemann surfaces $\Sigma$ (which are Stein manifolds by this Example), in which case it says that the corresponding inclusion of the space of holomorphic maps
induces an isomorphism on all homotopy groups, hence is a weak homotopy equivalence – reflecting the fact that non-compactness of the Riemann surfaces and absence of any asymptotic boundary condition provides a large supply of holomorphic functions.
Generalization to higher dimensional domain spaces:
Say that the degree of a rational map $f \;\colon\; \mathbb{C}P^{n_1} \to \mathbb{C}P^{n_2}$ between two complex projective spaces is the degree of the polynomials that define it.
In generalization of (1), say that a continuous map $f \;\colon\; \mathbb{C}P^{n_1} \to \mathbb{C}P^{n_2}$ between two complex projective spaces has degree $d \in \mathbb{N}$ (Rem. ) if this is the induced factor for pullback in their second integral ordinary cohomology (see here)
For $1 \leq n_1 \leq n_2$, and $d \in \mathbb{N}$, the inclusion
of the topological subspace of rational maps of algebraic degree $d$ into the mapping space of continuous functions of degree $d$ in the sense of (3) induces an isomorphism on integral ordinary homology in degrees
where $\lfloor - \rfloor$ denotes the integer floor of a rational number.
(Mostovoy 2006, Theorem 2, with corrected proof in Mostovoy 2012)
An analogous result for real projective spaces is in Adamaszek, Kozlowski & Yamaguchi 2008.
Prop. generalizes to the case that the codomain $\mathbb{C}P^{n_2}$ is allowed to be any smooth toric variety (Mostovoy & Munguia-Villanueva 2012, Kozlowski & Yamaguchi 2018).
The original theorem for maps from compact Riemann surfaces to projective spaces:
Further discussion:
Fred Cohen, Ralph Cohen, B. M. Mann, R. J. Milgram, The topology of rational functions and divisors of surfaces, Acta Math (1991) 166: 163 (doi:10.1007/BF02398886)
Eric M. Friedlander, H. Blaine Lawson, Section 5.C of: Duality Relating Spaces of Algebraic Cocycles and Cycles, Topology Volume 36, Issue 2, March 1997, Pages 533-565 (pdf)
Ralph L. Cohen, John D. S. Jones, Graeme B. Segal, Stability for holomorphic spheres and Morse theory, in: K. Grove, I. H. Madsen, E. K. Pedersen (eds.) Geometry and Topology: Aarhus, Contemporary Mathematics
Volume: 258 (2000) (arXiv:math/9904185, ISBN:978-0-8218-2158-9)
Yasuhiko Kamiyama, Remarks on spaces of real rational functions, The Rocky Mountain Journal of Mathematics Vol. 37, No. 1 (2007), pp. 247-257 (jstor:44239357)
Generalization to the case that the codomain is
… a Grassmannian:
… a toric variety:
… a flag manifold:
C. P. Boyer, B. M. Mann, J. C. Hurtubise, R. J. Milgram, The topology of the space of rational maps into generalized flag manifolds, Acta Mathematica. 1994 Mar 1;173(1):61-101 (doi:10.1007/BF02392569)
J. C. Hurtubise, Holomorphic maps of a Riemann surface into a flag manifold, J. Differential Geom. 43(1): 99-118 (1996) (doi:10.4310/jdg/1214457899)
Application to Skyrmions:
On maps between complex projective spaces:
corrected proof in:
On maps between real projective spaces:
On maps from real projective space to complex projective space:
and equivariantly:
On maps from complex projective space to smooth toric varieties:
Jacob Mostovoy, Erendira Munguia-Villanueva, Spaces of morphisms from a projective space to a toric variety, Revista Colombiana de Matematicas 48 1 (2014) (arXiv:1210.2795, published pdf)
Andrzej Kozlowski, Kohhei Yamaguchi, The homotopy type of spaces of rational curves on a toric variety, Topology and its Applications Volume 249, 1 November 2018, Pages 19-42 (doi:10.1016/j.topol.2018.06.006)
Last revised on July 29, 2021 at 12:52:04. See the history of this page for a list of all contributions to it.