# Euclidean triangles have no hot spots

### Abstract

We show that a second Neumann eigenfunction $u$ of a Euclidean triangle has at most one (non-vertex) critical point $p$, and if $p$ exists, then it is a non-degenerate critical point of Morse index $1$. Using this we deduce that

(1) the extremal values of $u$ are only achieved at a vertex of the triangle, and

(2) a generic acute triangle has exactly one (non-vertex) critical point and that each obtuse triangle has no (non-vertex) critical points.

This settles the `hot spots’ conjecture for triangles in the plane.

## Authors

Chris Judge

Indiana University, Bloomington, IN

Sugata Mondal

Tata Institute of Fundamental Research, Mumbai, India