= Unit-speed cubic time-derivative bispectrum
{title2=$B=3H^4\mathcal F/(8\epsilon^2k_1k_2k_3K^3)$}
For $H_{\rm int}=-\int d^3x\,a^3\epsilon\mathcal F\dot\zeta^3/H$, $a=-1/(H\tau)$ and stipulated modes $u_k=H(1+ik\tau)e^{-ik\tau}/\sqrt{4\epsilon k^3}$, six connected <Wick contractions> and $\int_{-\infty(1-i0)}^0\tau^2e^{iK\tau}d\tau=2i/K^3$ yield $B=3H^4\mathcal F/(8\epsilon^2k_1k_2k_3K^3)$. Here $\langle\zeta^3\rangle_c=(2\pi)^3\delta(\sum\mathbf k_i)B$. Using unstarred vertex modes with the external-before-vertex prefactor incorrectly reverses the sign. Non-unit sound speed also changes the quadratic modes, so this result is conditional on the specified unit-speed modes.
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