arXiv subjects
Publications and source records attributed to M. C. Crabb.
In this expository note, recent results of Kishimoto and Matsushita on triangulated manifolds are linked to the classical criterion on the normal Stiefel-Whitney classes for existence of an embedding of a smooth closed manifold into Euclidean space of given dimension. We also look back at Atiyah's K-theoretic condition for the existence of a smooth embedding.
Results of Macdonald and Dold from the 1960s and '70s expressing the Lefschetz numbers of symmetric powers of a self-map of a compact ENR in terms of the Lefschetz numbers of iterates of the map are extended using the notion of a Lefschetz-polynomial functor. Configuration spaces and Borsuk-Ulam symmetric products, as well as symmetric powers, are treated as examples of the general method.
The Poincaré-Hopf theorem for line fields, as described in a paper of Crowley and Grant, is interpreted as a special case of a Poincaré-Hopf theorem for $n$-valued sections of a vector bundle over a closed manifold of the same dimension.
Let $G$ be a finite group and $V$ a finite dimensional (non-zero) orthogonal $G$-module such that, for each prime $p$ dividing the order of $G$, the subspace of $V$ fixed by a Sylow $p$-subgroup of $G$ is non-zero and, if the dimension of $V$ is odd, has dimension greater than $1$. Using ideas of Avvakumov, Karasev, Kudrya and Skopenkov and work of Noakes on self-maps of sphere bundles, we show that, for any principal $G$-bundle $P\to X$ over a compact ENR $X$, there exists a $G$-map from $P$ to the unit sphere $S(V)$ in $V$.
An elementary result in point-set topology is used, with knowledge of the mod $2$ cohomology of real projective spaces, to establish classical results of Lebesgue and Knaster-Kuratowski-Mazurkiewicz, as well as the topological central point theorem of Karasev, which is applied to deduce results of Helly-Lovász, Bárány and Tverberg
We give a direct proof of a result due to Karasev (2008), Karasev-Matschke (2014) and Schnider-Soberón (2023). Given $m+1$ Borel probability measures on the space of affine hyperplanes in a real vector space $V$ of dimension $m+1$, there exist a line $L$ through the origin in $V$ and a point $v\in L$ such that at least half of the hyperplanes, as counted by any of the measures, meet or are parallel to each of the two closed rays in $L$ meeting at $v$.
Suppose that $f_1,\ldots ,f_m : S(V)\to R$ are $m$ ($\geq 1$) continuous functions defined on the unit sphere in a Euclidean vector space $V$ of dimension $m+1$ satisfying $f_i(-v)=-f_i(v)$ for all $v\in S(V)$. The classical Borsuk-Ulam theorem asserts that the image of the map $(f_1,\ldots ,f_m) :S(V)\to R^m$ contains $0=(0,\ldots ,0)$. Pursuing ideas in papers of Bárány, Hubard and Jéronimo (2008) and Frick and Wellner (2023), we show that a certain separation property will guarantee that the image contains an $m$-cube.
We establish a stable homotopy-theoretic version of a recent result of Farber and Weinberger on the fibrewise topological complexity of sphere bundles and prove, by closely parallel methods, a similar result for real, complex and quaternionic projective bundles. The symmetrized invariant introduced by Farber and Grant is also considered.
We describe a connective $K$-theory Borsuk--Ulam/Bourgin--Yang theorem for cyclic groups of order a power of a prime $p$. Consider two finite dimensional complex representations $U$ and $V$ of the cyclic group $Z /p^{k+1}$ of order $p^{k+1}$, where $k\geq 0$. For $0\leq l\leq k$, we write $V_l$ for the subspace of $V$ fixed by the cyclic subgroup of order $p^l$, and require that the fixed subspace, $V_{k+1}$, be zero and that $V_k$ be non-zero. Put $δ(V)=\sum_{l=0}^k p^l dim_C (V_l/V_{l+1})-(p^k-1)$. Then the zero-set of any $Z /p^{k+1}$-map $S(U) \to V$ from the unit sphere in $U$ (for some invariant inner product) has covering dimension greater than or equal to $2(dim_C U - δ(V)-1)$, if $dim_C U> δ(V)$.
The Intermediate Value Theorem is used to give an elementary proof of a Borsuk-Ulam theorem of Adams, Bush and Frick that, if $f: S^1\to R^{2k+1}$ is a continuous function on the unit circle $S^1$ in $C$ such that $f(-z)=-f(z)$ for all $z\in S^1$, then there is a finite subset $X$ of $S^1$ of diameter at most $π-π/(2k+1)$ (in the standard metric in which the circle has circumference of length $2π$) such the convex hull of $f(X)$ contains $0\in R^{2k+1}$.
Let $f: R^{m+1}\to R^{m+2^r}$, where $2^{r-1}\leq m+1 <2^r$, be a continuous map. Improving a recent result of Frick and Harrison, we show that there are $4$ points $x_0,\, x_1,\, y_0,\, y_1$ in $R^m$, which are distinct if $m+1\not=2^{r-1}$, and satisfy $x_0\not=x_1$, $y_0\not=y_1$, $\{ x_0, x_1\} \not=\{ y_0,y_1\}$ if $m+1=2^{r-1}$, such that the vectors $f(x_1)-f(x_0)$ and $f(y_1)-f(y_0)$ are parallel.
Let $G$ be a compact Lie group and let $U$ and $V$ be finite-dimensional real $G$-modules with $V^G=0$. A theorem of Marzantowicz, de Mattos and dos Santos estimates the covering dimension of the zero-set of a $G$-map from the unit sphere in $U$ to $V$ when $G$ is an elementary elementary abelian $p$-group for some prime $p$ or a torus. In this note, the classical Borsuk--Ulam theorem will be used to give a refinement of their result estimating the dimension of that part of the zero-set on which an elementary abelian $p$-group $G$ acts freely or a torus $G$ acts with finite isotropy groups.