(Left) The family of densities p(v;a,b=1)=𝒢(v;a,b/a) with the same mean 〈v〉=b=1. Small values of a (for a<1) enforce sparser representations, and large values of a≈100 tie all values to be close to a nonzero mean (nonsparse representation). To model missing data, that is, when some of the are not observed, we define a mask matrix , the same size as where , if is missing and otherwise (see Appendi