The Borsuk-Ulam property for homotopy classes of selfmaps of surfaces of Euler characteristic zero

Abstract

Let M and N be topological spaces such that M admits a free involution $\τ$. A homotopy class $\beta$ $\in$ [M, N ] is said to have the Borsuk-Ulam property with respect to $\τ$ if for every representative map f : M $\to$ N of $\beta$, there exists a point x $\in$ M such that f ($\τ$ (x)) = f (x). In the case where M is a compact, connected manifold without boundary and N is a compact, connected surface without boundary different from the 2-sphere and the real projective plane, we formulate this property in terms of the pure and full 2-string braid groups of N , and of the fundamental groups of M and the orbit space of M with respect to the action of $\τ$. If M = N is either the 2-torus T^2 or the Klein bottle K^2 , we then solve the problem of deciding which homotopy classes of [M, M ] have the Borsuk-Ulam property. First, if $\τ$ : T^2 $\to$ T^2 is a free involution that preserves orientation, we show that no homotopy class of [T^2 , T^2 ] has the Borsuk-Ulam property with respect to $\τ$. Secondly, we prove that up to a certain equivalence relation, there is only one class of free involutions $\τ$ : T^2 $\to$ T^2 that reverse orientation, and for such involutions, we classify the homotopy classes in [T^2 , T^2 ] that have the Borsuk-Ulam property with respect to $\τ$ in terms of the induced homomorphism on the fundamental group. Finally, we show that if $\τ$ : K^2 $\to$ K^2 is a free involution, then a homotopy class of [K^2 , K^2 ] has the Borsuk-Ulam property with respect to $\τ$ if and only if the given homotopy class lifts to the torus.