Primordial-to-angular bispectrum projection ($b_{{\ell }_1{\ell }_2{\ell }_3}=(2/\pi {)}^3\int x^{2}dx{\prod }_i[k_i^{2}d{k}_i{j}_{{\ell }_i}(k_{i}x)g_{{\ell }_i}(k_i)]B$)

Linear cosmological transfer maps the primordial bispectrum to the reduced CMB bispectrum. A Fourier representation of the momentum delta function and three Rayleigh plane-wave expansions separate the radial transfer integrals from a Gaunt integral. The six angular factors and three Fourier measures give $(4\pi {)}^6/(2\pi {)}^9=(2/\pi {)}^3$. The observer-position phase cancels by momentum conservation; the remaining radial position is an auxiliary integration variable.