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Faster Projection-free Convex Optimization over the Spectrahedron
Dan Garber · Dan Garber

Tue Dec 06 09:00 AM -- 12:30 PM (PST) @ Area 5+6+7+8 #45
Minimizing a convex function over the spectrahedron, i.e., the set of all $d\times d$ positive semidefinite matrices with unit trace, is an important optimization task with many applications in optimization, machine learning, and signal processing. It is also notoriously difficult to solve in large-scale since standard techniques require to compute expensive matrix decompositions. An alternative, is the conditional gradient method (aka Frank-Wolfe algorithm) that regained much interest in recent years, mostly due to its application to this specific setting. The key benefit of the CG method is that it avoids expensive matrix decompositions all together, and simply requires a single eigenvector computation per iteration, which is much more efficient. On the downside, the CG method, in general, converges with an inferior rate. The error for minimizing a $\beta$-smooth function after $t$ iterations scales like $\beta/t$. This rate does not improve even if the function is also strongly convex. In this work we present a modification of the CG method tailored for the spectrahedron. The per-iteration complexity of the method is essentially identical to that of the standard CG method: only a single eigenvecor computation is required. For minimizing an $\alpha$-strongly convex and $\beta$-smooth function, the \textit{expected} error of the method after $t$ iterations is: $O\left({\min\{\frac{\beta{}}{t} ,\left({\frac{\beta\sqrt{\rank(\X^*)}}{\alpha^{1/4}t}}\right)^{4/3}, \left({\frac{\beta}{\sqrt{\alpha}\lambda_{\min}(\X^*)t}}\right)^{2}\}}\right)$. Beyond the significant improvement in convergence rate, it also follows that when the optimum is low-rank, our method provides better accuracy-rank tradeoff than the standard CG method. To the best of our knowledge, this is the first result that attains provably faster convergence rates for a CG variant for optimization over the spectrahedron. We also present encouraging preliminary empirical results.

Author Information

Dan Garber (Technion)
Dan Garber (Toyota Technological Institute at Chicago)

Dan's research interests lie in the intersection of machine learning and continuous optimization. Dan's main focus is on the development of efficient algorithms with novel and provable performance guarantees for basic machine learning, data analysis, decision making and optimization problems. Dan received both his Ph.D and his M.Sc degrees from the Technion - Israel Institute of Technology, where he worked under the supervision of Prof. Elad Hazan. Before that, Dan completed his bachelor's degree in computer engineering, also in the Technion.

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