It is known that operator splitting methods based on Forward Backward Splitting (FBS), Douglas-Rachford Splitting (DRS), and Davis-Yin Splitting (DYS) decompose a difficult optimization problems into simpler subproblem under proper convexity and smoothness assumptions. In this paper, we identify an envelope (an objective function) whose gradient descent iteration under a variable metric coincides with DYS iteration. This result generalizes the Moreau envelope for proximal-point iteration and the envelopes for FBS and DRS iterations identified by Patrinos, Stella, and Themelis.Based on the new envelope and the Stable-Center Manifold Theorem, we further show that, when FBS or DRS iterations start from random points, they avoid all strict saddle points with probability one. This result extends the similar results by Lee et al. from gradient descent to splitting methods.
4When h = 0, (5) simplifies to Douglas-Rachford Splitting iteration,When g = 0, P γg reduces to Id and thus (5) simplifies towhich is Forward-Backward Splitting iteration slightly generalized by including the linear operator L.When f = 0, P γf reduces to Id and (5) simplifies to Backward-Forward Splitting,When f = g = 0, (5) simplifies to gradient descent iteration
Derivation of envelopeNow we show that, (5) can be written as gradient descent iteration of an envelope function under the following assumption.Assumption 1.