Flattened bispectrum configuration ($k_3=k_1+k_2$)

A flattened primordial bispectrum configuration lies at the collinear boundary of the allowed momentum-triangle domain. For a cubic time-derivative curvature bispectrum, it is finite rather than singular. Its equal-short-side value $k_1=k_2=k_3/2$ gives $k_1{k}_2{k}_3/(k_1+k_2+k_3{)}^3=1/32$, compared with $1/27$ at an equilateral bispectrum configuration.