M.  Heiler and Schnörr, C., ?Learning Sparse Representations by Non-Negative Matrix Factorization and Sequential Cone Programming?, J. Mach. Learning Res., vol. 7, pp. 1385?1407, 2006.

M.  Heiler and Schnörr, C., ?Learning Sparse Image Codes by Convex Programming?, in Proc. Tenth IEEE Int. Conf. Computer Vision (ICCV'05), Beijing, China, 2005, pp. 1667-1674.