Cofiniteness for Twisted Fusion Products in Vertex Operator Algebra Theory

Let $V$ be a vertex operator algebra equipped with two commuting finite-order automorphisms $g_1$ and $g_2$, and set $g_3=g_1{g}_2$. For $k=1,2,3$, let $W^k$ be a $g_k$-twisted $V$-module. Assuming that $W^1$ and $W^2$ are $C_1$-cofinite and that there exists a surjective twisted logarithmic intertwining operator of type $\left(\genfrac{}{}{0px}{}{W^3}{W^1{W}^2}\right)$, we prove that $W^3$ is also $C_1$-cofinite. The cofiniteness follows from the finite-dimensionality of the solution space of an associated complex-coefficient linear differential equation. As an application, under the condition of $C_1$-cofiniteness, we establish the finiteness of the fusion rules and construct the fusion product.