  We provide a mirror symmetry theorem in a range of cases where the
state-of-the-art techniques relying on concavity or convexity do not apply.
More specifically, we work on a family of FJRW potentials named after Fan,
Jarvis, Ruan, and Witten's quantum singularity theory and viewed as the
counterpart of a non-convex Gromov--Witten potential via the physical LG/CY
correspondence. The main result provides an explicit formula for Polishchuk and
Vaintrob's virtual cycle in genus zero. In the non-concave case of the
so-called chain invertible polynomials, it yields a compatibility theorem with
the FJRW virtual cycle and a proof of mirror symmetry for FJRW theory.
