SUMMARYThe article explores the relationship between Sobolev gradients and H −1 mixed methods for a variety of partial differential equations (PDEs) from image processing. A first-order system least-squares problem is used to introduce the method and compare the Euclidean with the Sobolev gradient. The standard two-term decomposition of an image as f = u + v with u ∈ H 1 and v ∈ L 2 = H 0 yields a second-order linear PDE, while minimizing other L p norms give nonlinear PDEs. Finally, a three-term decomposition f = u + v + w with u ∈ H 1 , v ∈ H −1 , w ∈ H 0 requires the solution of a fourth-order system with the biharmonic operator.