Sachs-Wolfe projection of a constant bispectrum ($b_{{\ell }_1{\ell }_2{\ell }_3}=\frac{4{\pi }^4{f}_{\mathrm{NL}}{\Delta }_{\zeta }^4}{125{\prod }_i(2{\ell }_i+1)}\left(\frac{1}{L+3}+\frac{1}{L}\right),L={\sum }_i{\ell }_i$)

For a constant primordial bispectrum and large-angle transfer function ${\Delta }_{\ell }(k)=j_{\ell }(kR)/5$, the spherical Bessel product integral reduces the radial projection to ${\int }_0^1{r}^{L+2}dr+{\int }_1^{\infty }{r}^{-L-1}dr$. For $L>0$ this equals $1/(L+3)+1/L$, giving the displayed reduced CMB bispectrum. All powers of the distance $R$ cancel. Under common large-multipole scaling it behaves as ${\ell }^{-4}$ at fixed shape; this is angular scale invariance, not a constant angular bispectrum. The all-monopole case $L=0$ has a logarithmically divergent radial tail and is excluded.