A spinorial analogue of Aubin's inequality

Abstract

Let $(M,g,\si)$ be a compact Riemannian spin manifold of dimension $\geq 2$. For any metric $\tilde g$ conformal to $g$, we denote by $\tilde\lambda$ the first positive eigenvalue of the Dirac operator on $(M,\tilde g,\si)$. We show that $$\inf_{\tilde{g} \in [g]} \tilde\lambda \Vol(M,\tilde g)^{1/n} \leq (n/2) \Vol(S^n)^{1/n}.$$ This inequality is a spinorial analogue of Aubin's inequality, an important inequality in the solution of the Yamabe problem. The inequality is already known in the case $n \geq 3$ and in the case $n = 2$, $\ker D=\{0\}$. Our proof also works in the remaining case $n=2$, $\ker D\neq \{0\}$. With the same method we also prove that any conformal class on a Riemann surface contains a metric with $2\tilde\lambda^2\leq \tilde\mu$, where $\tilde\mu$ denotes the first positive eigenvalue of the Laplace operator.