“…The hard thresholding methods have widely been studies in the area of compressed sensing or sparse approximation [5,6,7,29,30,4]. The latest development and applications of these methods can be found in [8,9,34,46,52,54,48]. Although the sparse optimization problems like (1) arising from compressed sensing are usually NP-hard [42], it does not prohibit the fast development of various computational methods for such problems.…”