Backward-Forward Least Angle Shrinkage for Sparse Quadratic Optimization

In compressed sensing and statistical society, dozens of algorithms have been developed to solve `1 penalized least square regression, but constrained sparse quadratic optimization (SQO) is still an open problem. In this paper, we propose backward-forward least angle shrinkage (BF-LAS), which provides a scheme to solve general SQO including sparse eigenvalue minimization. BF-LAS starts from the dense solution, iteratively shrinks unimportant variables’ magnitudes to zeros in the backward step for minimizing the `1 norm, decreases important variables’ gradients in the forward step for optimizing the objective, and projects the solution on the feasible set defined by the constraints. The importance of a variable is measured by its correlation w.r.t the objective and is updated via least angle shrinkage (LAS). We show promising performance of BF-LAS on sparse dimension reduction.