Sparse regression as a sparse eigenvalue problem

We extend the l₀-norm ldquosubspectralrdquo algorithms developed for sparse-LDA (Moghaddam, 2006) and sparse-PCA (Moghaddam, 2006) to more general quadratic costs such as MSE in linear (or kernel) regression. The resulting ldquosparse least squaresrdquo (SLS) problem is also NP-hard, by way of its equivalence to a rank-1 sparse eigenvalue problem. Specifically, for minimizing general quadratic cost functions we use a highly-efficient method for direct eigenvalue computation based on partitioned matrix inverse techniques that leads to times10³ speed-ups over standard eigenvalue decomposition. This increased efficiency mitigates the O(n⁴) complexity that limited the previous algorithmspsila utility for high-dimensional problems. Moreover, the new computation prioritizes the role of the less-myopic backward elimination stage which becomes even more efficient than forward selection. Similarly, branch-and-bound search for exact sparse least squares (ESLS) also benefits from partitioned matrix techniques. Our greedy sparse least squares (GSLS) algorithm generalizes Natarajanpsilas algorithm (Natarajan, 1995) also known as order-recursive matching pursuit (ORMP). Specifically, the forward pass of GSLS is exactly equivalent to ORMP but is more efficient, and by including the backward pass, which only doubles the computation, we can achieve a lower MSE than ORMP. In experimental comparisons with LARS (Efron, 2004), forward-GSLS is shown to be not only more efficient and accurate but more flexible in terms of choice of regularization.