[en] A compact correlation-function expansion is developed for n-order optical susceptibilities in the frequency domain using the Keldysh-Schwinger loop. By not keeping track of the relative time ordering of bra and ket interactions at the two branches of the loop, the resulting expressions contain only n+1 basic terms, compared to the 2ⁿ terms required for a fully-time-ordered density matrix description. Superoperator Green's function expressions for χ⁽ⁿ⁾ derived using both expansions reflect different types of interferences between pathways. These are demonstrated for correlation-induced resonances in four-wave-mixing signals