Neumann eigenvalue sums on triangles are (mostly) minimal for   equilaterals
  We prove that among all triangles of given diameter, the equilateral triangle minimizes the sum of the first $n$ eigenvalues of the Neumann Laplacian, when $n \geq 3$.   The result fails for $n=2$, because the second eigenvalue is known to be minimal for the degenerate acute isosceles triangle (rather than for the equilateral) while the first eigenvalue is 0 for every triangle. We show the third eigenvalue is minimal for the equilateral triangle. 
R. S. Laugesen, Z. C. Pan and S. S. Son
math.AP math-ph math.MP