Injectivity radius lower bound of convex sum of tame Riemannian metrics and applications to symplectic topology

Abstract

Motivated by the aspect of large-scale symplectic topology, we prove that for any pair $g_0, \, g_1$ of complete Riemannian metrics of bounded curvature and \emph{of injectivity radius bounded away from zero}, the convex sum $g_s: = (1-s ) g_0 + s g_1$ also has bounded curvature depending only on the curvature bounds $\|R_{g_i}\|_{C^0}$ of $g_0$ or $g_1$, and that the injectivity radii of $g_s$ have uniform lower bound depending only on the derivative bounds $\|R_{g_i}\|_{C^1} = \|R_{g_i}\|_{C^0} + \|DR_{g_i}\|_{C^0}$. A main technical ingredient to establish the injectivity radius lower bound is an application of the quantitative inverse function theorem. Using these estimates, we prove that each quasi-isometry class of tame metrics is convex and so contractible in strong $C^r$ topology for all finite regularity class of $3 \leq r < \infty$. Using this Riemannian geometry result, we prove that the set of $C^3$-tame almost complex structures inside the same quasi-isometry class associated to the symplectic form $\omega$ is contractible.