Everything is Vecchia: Unifying low-rank and sparse inverse Cholesky approximations

The partial pivoted Cholesky approximation accurately represents matrices that are close to being low-rank. Meanwhile, the Vecchia approximation accurately represents matrices with inverse Cholesky factors that are close to being sparse. What happens if a partial Cholesky approximation is combined with a Vecchia approximation of the residual? This paper shows how the sum is exactly a Vecchia approximation of the original matrix with an augmented sparsity pattern. Thus, Vecchia approximations subsume a class of existing matrix approximations and have broad applicability.