Note on the Equivalence of Orthogonalizing EM and Proximal Gradient Descent

Xiong et al. (2016) develop a method called orthogonalizing EM (OEM) to solve penalized regression problems for tall data. While OEM is developed in the context of the EM algorithm, we show that it is, in fact, an instance of proximal gradient descent, a popular first-order convex optimization algorithm.