In this paper, we derive a Pogorelov type interior C² estimate for the Hessian quotient equation ₙ ₖ (D²u) =f. As an application, we show that convex viscosity solutions are regular for k n-3 if u C^1, with >1-2n-k or u W^2, p with p (n-1) (n-k) 2. Both exponents are sharp in view of the example in arXiv: 2401. 12229.
Lu et al. (Thu,) studied this question.