A surgery formula for the second Yamabe invariant

TL;DR

This paper establishes a surgery formula for the second Yamabe invariant, showing it remains bounded below under certain topological modifications, and explores implications for the topology of manifolds.

Contribution

It proves a lower bound for the second Yamabe invariant after surgery, extending understanding of its stability under topological changes.

Findings

The second Yamabe invariant is preserved under certain surgeries.

A positive constant bounds the invariant from below after surgery.

Topological consequences are derived from the invariance properties.

Abstract

Let $(M,g)$ be a compact Riemannian manifold of dimension $n\geq 3$. For a metric $g$ on $M$, we let $\la_2(g)$ be the second eigenvalue of the Yamabe operator $L_g:= \frac{4(n-1)}{n-2} \Delta_g + \scal_g$. Then, the second Yamabe invariant is defined as $$ \si_2(M) \definedas \sup \inf_{h \in [g]} \la_2(h) \Vol(M,h)^{2/n}. $$ where the supremum is taken over all metrics $g$ and the infimum is taken over the metrics in the conformal class $[g]$. Assume that $\si_2(M)>0$. In the spirit of \cite{ammann.dahl.humbert:08}, we prove that if $N$ is obtained from $M$ by a $k$-dimensional surgery ($0 \leq k \leq n-3$), there exists a positive constant $\Lambda_n$ depending only on $n$ such that $\si_2(N) \geq \min(\sigma_2(M), \Lambda_n)$. We then give some topological conclusions of this result.