Solution
= Solution
The covariance of a centered <multivariate Gaussian distribution> is the inverse of its quadratic kernel. Writing $A(q)=a+\kappa q^2+\gamma q^4$, one finds
$$
G(q)^{-1}
=\frac1{\nu A(q)-\lambda^2q^2}
\begin{pmatrix}
\nu&-i\lambda q\\
i\lambda q&A(q)
\end{pmatrix}.
$$
Consequently the composition <static structure factor> is
$$
\boxed{\langle\phi(q)\phi(-q)\rangle
=\frac1{a+\widetilde\kappa q^2+\gamma q^4}.}
$$