Non-central sections of the regular n-simplex

Abstract

We show that the maximal non-central hyperplane sections of the regular n-simplex of side-length sqrt 2 at a fixed distance t to the centroid are those parallel to a face of the simplex, if $\sqrt{(n-2)/(3(n+1))}<t<\sqrt{(n-1)/(2(n+1))}$ and $n>4$. For $n=4$, the same is true in a slightly smaller range for t. This adds to a previous result for $\sqrt{(n-1)/(2(n+1))}<t<\sqrt{n/(n+1)}$. For $n=2,3$, we determine the maximal and the minimal sections for all distances t to the centroid.