= Solution
For nonzero momentum, the Fourier transform of the local <bias expansion> is
$$
\delta_g(\mathbf k)=b_1\delta(\mathbf k)
+\frac{b_2}{2}\int\frac{d^3q}{(2\pi)^3}
\delta(\mathbf q)\delta(\mathbf k-\mathbf q).
$$
The subtracted variance contributes only at $\mathbf k=0$. Because $\delta$ is a <Gaussian random field>, its three-point function vanishes and its four-point function factorizes by <Wick theorem>. At leading order one of the three galaxy fields supplies the quadratic term and the other two supply linear terms. The two cross-contractions cancel the factor $1/2$, giving
$$
\boxed{
\langle\delta_g(\mathbf k_1)\delta_g(\mathbf k_2)
\delta_g(\mathbf k_3)\rangle
=b_1^2b_2(2\pi)^3\delta^{(3)}\!\left(\sum_a\mathbf k_a\right)
\left[P(k_1)P(k_2)+P(k_2)P(k_3)+P(k_3)P(k_1)\right]}.
$$
Thus even Gaussian matter fluctuations acquire a nonzero <galaxy bispectrum> through <local quadratic galaxy bias>.
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