Equilateral bispectrum configuration ($k_1=k_2=k_3$)

An equilateral primordial bispectrum configuration has equal external wavenumbers. For a cubic time-derivative curvature bispectrum, $S\propto k_1{k}_2{k}_3/(k_1+k_2+k_3{)}^3$ is maximized here at fixed perimeter by the arithmetic-geometric mean inequality.