 Solution

For a proper convex function $E$ that is lower semicontinuous on a Hilbert space, its proximal operator at scale $\gamma >0$ isThe minimizer is unique because the objective is strongly convex; existence follows from the closed proper convex functional and the coercive quadratic, using an affine lower bound. The subgradient optimality condition is $0\in x-z+\gamma \partial E(x)$, which is exactly the displayed resolvent of a monotone operator relation. Setting $\gamma =1$ gives the unscaled proximity or resolvent operator requested here. This is a nonlinear resolvent of the subdifferential, distinct from the spectral resolvent of a linear operator.