
# The Cyclic Proximal Point Algorithm

This algorithm computes the cyclic or randomized proximal point algorithm on a manifold $\M$ for given input data $x\colon \Grid \to \M$, where $\Grid$ denotes the data domain —usually a signal or pixel grid, but necessarily on some array form—, a set $f_i$ of proximal maps, and a stopping criterion.

Given a function $F\colon\M\to\mathbb R$ this algorithm aims to find a minimizer

so in a sense it is similar to a gradient descent method, but it is also applicable if $F$ is not differentiable and there is no need to determine a step size; choosing the $lambda_k$ described in the following determine merely the speed of convergence.

If we know for $F$, that there exists a decomposition of $F$ into $c\in\mathbb N$ atoms

where each of the $f_i$ is assumed to have a proximal map, that is either known in closed form or otherwise easily (approximately to be computed, i.e.

see [1] for an introduction in Euclidead space $\M=\R$. The cyclic proximal point (CPP) algorithm was introduced on manifolds in [2]. It consists of applying the proximal maps iteratively

where we denote the inner (one cycle) iterations by a fractional index.

### Parameters

While the M denotes a manifold class and x denotes the initial data $x_0$ both remaining parameters are functionals. The easier one is the stoppingCriterion = @(x,xold,lambda,iter) with the for parameters $x^{(k)}$, $x^{(k-1)}$, $\lambda_k$ and the current iteration $k$. The itertion is running while the functional returns false, i.e. the stoppingCriterion is not fulfilled and stops after a cycle if it returns true. The easiest is of course just depending on the iteration: @(x,xold,lambda,iter) iter>1999 will stop after 2000 iterations.

The varibale proximalMaps is a cell array containing the proximal maps as anonymous functions. More precisely each entry of proximalMaps is an anonymous function

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fi = @(x,lambda) ...


evaluating the corresponding proximal map $\prox_{\lambda f_i}(x)$ of $f_i$. For a list, see below

### Optional Parameters

lambdaIterate
a functional @(iter) returning a value for $\lambda_k$
lambda
easiest way to set the lambda iterate: defines the lambdaIterate as $\lambda_k = \frac{\lambda}{k}$
Order
if not specified, the above cyclic order is used. Other possibilities are 'permute' which permutes the order of the proximal maps in each cycle randomly or 'random' which choses one proximal map at random per iteration to be evaluated.
Debug
If set to an anonymous functional depending on @(x,xold,iter) this is called in each cycle, for example to produce text output or save iteration data to a file.
Record
If set to a functional @(x,iter) returning a column vector, and calling the cyclicProximalPoint with a second return value, this function records these column vectors in a matrix returned in the second return value recData.

### Matlab Documentation

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% CyclicProximalPoint(M,f,proximalMaps,stoppingCriterion)
%    Compute the cyclic proximal point algorithm starting with data f,
%    cyclic through the proximal points
%  Input
%   M                 : a manifold
%   x                 : given (initial) data
%   proximalMaps      : cell arrays of functionals computing the proximal
%                       maps, i.e. can be called as prox = @(x,lambda)
%   stoppingCriterion : a functional @(x,xold,lambda,iter) as a stopping
%                       criterion based on x, the last iterate xold,
%                       lambda,
%                       that indicates, then to stop, i.e., is 1 for stops.
%
% OPTIONAL
%  'lambdaIterate' : (@ (iter)) a functional @(iter) computing the
%                        iterates value of lambda
%  'lambda'        : if no iterate (see above) is given, this value is used
%                        to set the iterate to @(iter) lambda/iter
%  'Order'         : ('') the standard order is ascending in proximalMaps,
%                      'random' takes a random proximal map each iteration
%                       (note that then one iteration is only _one_
%                       proximal map evaluated)
%                      'permute' keeps a cyclic manner but permutes the
%                       order for each cycle anew.
%   Debug          : ([]) a function @ (x,xold,iter) producing debug
%                       output
%   Record         : (@(x,iter)) a function returning a column vector, if
%                       there's a second return value and this function is
%                       given, data is recorded in an array and returned
%
% OUTPUT
%   y       : result of the CPP algorithm
%   recData : recorded data array of the by the record-function
% ---
% Manifold-Valued Image Restoration Toolbox 1.0
% R. Bergmann ~ 2018-01-22 | 2018-03-04