Minimal isometric immersions of flat n-tori into spheres

Abstract

In 1985, Bryant established that a flat $2$-torus admits a minimal isometric immersion into some round sphere if and only if a certain rationality condition is satisfied. We show that when $n\geq 3$, the rationality criterion is no longer a necessary, but a sufficient condition for a flat $n$-torus to admit minimal isometric immersions into spheres. We also derive an upper bound for the algebraic irrationality degree of such immersions. When $n=3$, this bound is sharp and explicit embedded examples are provided respectively for each possible degree. Moreover, by constructing a family of non-homogeneous minimal flat $3$-tori, we show that minimal isometric immersions (embeddings) of flat $n$-tori are not necessarily homogeneous when $n \geq 3$. In addition, we establish a deformation theorem that every flat $n$-torus admitting a minimal isometric spherical immersion can be isometrically, minimally and homogeneously immersed into a sphere of dimension at most $n^2+n-1$.