Given an overcomplete dictionary matrix D\\ in\\ mathbb ^ that contains K signal-atoms, a signal y\\ in\\ mathbb ^ can be represented as a linear combination of these atoms.
Given an overcomplete dictionary matrix D\\ in\\ mathbb ^ that contains K signal-atoms, here each column of D is consider an signal-atom. A signal y\\ in\\ mathbb ^ can be represented as a linear combination of these atoms.