Scalar-Tracking SAV Schemes with Pullback Corrections for Gradient Flows

Abstract: The scalar auxiliary variable (SAV) method constructs linear, unconditionally energy-stable time discretizations of gradient flows. In a first-order SAV step, eliminating the auxiliary variable shows that the state equation is a semi-implicit update augmented by a rank-one positive semidefinite correction from the previous nonlinear force. The multiple-SAV (MSAV) method produces this correction componentwise, yielding a correction of rank up to the number of energy components. This separates two mechanisms usually coupled in MSAV: the number of scalar variables tracking the nonlinear energy and the rank of the correction applied to the state equation. We introduce a pullback-corrected SAV (PB-SAV) family that keeps a single scalar auxiliary variable but replaces the rank-one SAV correction by the pullback correction induced by an admissible component decomposition. The correction remains positive semidefinite, has rank at most the number of components, and may change from step to step without changing the scalar energy tracker. We prove modified-energy dissipation laws for fixed and step-dependent decompositions, derive a refinement identity whose gain is an explicit weighted variance, and give a Sherman-Morrison-Woodbury implementation of the low-rank perturbation of the standard semi-implicit solve. We also show, in finite dimensions, that the pullback correction is the Gauss-Newton matrix of a least-squares representation of the nonlinear energy. Numerical experiments on finite-dimensional gradient flows, Allen-Cahn dynamics, and nonlocal Cahn-Hilliard models illustrate regimes in which PB-SAV mainly changes the first-order error constant and regimes in which it substantially improves trajectory accuracy.