The sigma invariant of the $n$ torus, the K3 surface, and Euclidean and elliptic 3d manifolds

Abstract

On the space of isometric embeddings $f_g$ of metrics $g$ on a manifold $M^n$ into the standard $(\mb{S}^{\tn=\tn(n)},\tg)$, we consider the total exterior scalar curvature $\Theta_{f_g}(M)$, and squared $L^2$ norm of the mean curvature vector $\Phi_{f_g}(M)$ and second fundamental form $\Pi_{f_g}(M)$ functionals of $f_g$, respectively. Then $\mc{W}_{f_g}(M) =(1-\delta_{n,1})(n/(n-1)) \Theta_{f_{g}}(M) + \Phi_{f_{g)}}(M)$ and $\mc{D}_{f_g}(M)=(1-\delta_{n,1}) (1/(n-1))\Theta_{f_g}(M)+\Pi_{f_{g)}}(M)$ are functionals intrinsically defined in the space of metrics in the conformal class of $g$, and $\mc{S}_g(M):=\int s_g d\mu_g=\mc{W}_{f_g}(M)- \mc{D}_{f_g}(M)$. We extend the notions of $\sigma$ invariant and Kazdan-Warner type to manifolds of dimension $n\geq 1$. $M$ is a manifold of type II if, and only if, it admits a Ricci flat metric $g$ with minimal isometric embedding $f_g$ that minimizes $\mc{W}_{f_{g'}}(M)$ and $\mc{D}_{f_{g'}}(M)$ among metrics $g'$ in conformal classes $[g']$ with scalar flat representatives. We show that the torus $T^n$, the K3 surface, and any Euclidean 3d manifold are manifolds of Kazdan-Warner type II, exhibiting in each case the canonical Ricci flat $g$ that realizes the vanishing $\sigma$ invariant and said minimal value $\mc{W}_{f_g}(M)= \mc{D}_{f_g}(M)$, with Euclidean 3d manifolds of isomorphic $\pi_1$ being diffeomorphic iff the values of $\mc{W}_{f_g}(M)$ for their canonical $g$s are the same. An elliptic 3d manifold $(M,\Gamma_M)$ of underlying group $\pi_1(M)\cong \Gamma_M \subset \mb{S}\mb{O}(4)$ has $\sigma(M) =6(2\pi^2)^{\frac{2}{3}}/|\pi_1(M)|^{\frac{2}{3}}$, and if $(M,\Gamma_M)$ and $(M',\Gamma_{M'})$ are two of them of isomorphic $\pi_1$, $M$ is diffeomorphic to $M'$ iff the spaces of $\Gamma_M$ and $\Gamma_{M'}$ invariant homogeneous spherical harmonics of degree $|\pi_1|$ are the same.