Metrics and norms used for obtaining sparse solutions to underdetermined   Systems of Linear Equations
  This paper focuses on defining a measure, appropriate for obtaining optimally sparse solutions to underdetermined systems of linear equations.* The general idea is the extension of metrics in n-dimensional spaces via the Cartesian product of metric spaces. *The following work done, was within the completion of my master thesis titled "Algorithms for the computation of sparse solutions of undefined systems of equations" at the department of Mathematics, University of Athens which was assigned to me in association with the department of Informatics and Telecommunications, National and Kapodistrian University of Athens. 
Leoni Dalla, George K. Papageorgiou
math.OC math.FA