Half quadratic regularization
This method is based on the introduction of an auxiliary variable b
related to the presence of discontinuities. If the regularizing
function 
verifies some properties (cf. [3][5]), then
we have :
with 
strictly convex and deduced from the regularizing function. The
auxiliary variable is :
We get the following functional
which is convex with respect to (wrt) b and locally quadratic wrt 
(according to
the definition of W ).