#Positive Definte Least Squares Problem
min_X a*trace(X'*X) - trace(Y'*X)
subject to X is positive (semi)-definite matrix
X,Y are symmetric n-by-n matrices
using Accelerated Projected Gradient(APG) algorithm
@author: Junhao Hua
@email: huajh7@gmail.com
Latest update: 2015/4/16