I. Introduction
Single image superresolution (SISR) aims to recover a high-resolution (HR) image from a single low-resolution (LR) image, which is widely used in medical image [1], satellite image [2], and remote sensing image [3]. However, SISR has a well-known ill-posed problem, which the detail information of LR image loses. Many traditional methods are proposed to solve this problem, such as bicubic interpolation [4], maximum a posterior [5], neighbor embedding [6], [7], sparse representation [8], [9], and so on.