= Primordial-to-angular bispectrum projection
{title2=$b_{\ell_1\ell_2\ell_3}=(2/\pi)^3\int x^2dx\,\prod_i[k_i^2dk_i\,j_{\ell_i}(k_ix)g_{\ell_i}(k_i)]B$}
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.
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