I. Introduction
A dataset is called non-negative if only non-negative values are allowed in the data expression. Examples of non-negative data include images and power spectra of signals. Often, the non-negative data can be expressed as a non-negative matrix , which may have a lower rank than would be indicated by its dimensions. The Non-negative Matrix Factorization (NMF), finds a compact representation of as , where and are non-negative matrices with smaller dimensions.