Primordial bispectrum shape function ($S(k_1,k_2,k_3)\propto (k_1{k}_2{k}_3{)}^{2}B(k_1,k_2,k_3)$)

For a scale-invariant primordial bispectrum, the displayed shape function is invariant under common rescaling of all three wavenumbers. It isolates triangle dependence from the overall power spectrum amplitude. It indicates intrinsic shape preference but alone does not determine observational signal-to-noise, which also depends on transfer functions, covariance and survey weights.