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arXiv 1502.05666math.OC
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光滑强凸插值与一阶方法的精确最坏情况性能

Smooth Strongly Convex Interpolation and Exact Worst-case Performance of First-order Methods

Adrien B. Taylor, Julien M. Hendrickx, François Glineur

摘要:

我们表明,精确的最坏情况下性能的固定步骤的一阶方法的光滑(可能强)凸函数的无约束优化可以通过求解凸规划。寻找黑箱一阶方法的最坏情况下的性能制定为一组光滑(强)凸函数和初始条件的最优化问题。给出了光滑(强)凸插值的封闭形式的充要条件,并给出了这些函数的有限表示。这使我们能够重新制定的最坏情况下的性能估计问题作为一个等价的有限维独立的半定最优化问题,其精确解可以恢复到数值精度。这个性能估计问题的最优解提供了最坏情况下的性能界限和显式函数匹配,因为我们的光滑(强)凸插值过程是建设性的。我们的作品建立在那些Drori和Teboulle在[数学。145(1-2),2014],其引入并解决了光滑凸函数的性能估计问题的松弛。我们将我们的方法应用到不同的固定步长的一阶方法与几个性能标准,包括目标函数精度和梯度范数。在光滑凸和光滑强凸情形下,我们分别给出了固定步长梯度法、快速梯度法和优化梯度法的最坏情况的数值界,并推导了梯度法的最优步长的严格估计.

英文摘要:

We show that the exact worst-case performance of fixed-step first-order methods for unconstrained optimization of smooth (possibly strongly) convex functions can be obtained by solving convex programs. Finding the worst-case performance of a black-box first-order method is formulated as an optimization problem over a set of smooth (strongly) convex functions and initial conditions. We develop closed-form necessary and sufficient conditions for smooth (strongly) convex interpolation, which provide a finite representation for those functions. This allows us to reformulate the worst-case performance estimation problem as an equivalent finite dimension-independent semidefinite optimization problem, whose exact solution can be recovered up to numerical precision. Optimal solutions to this performance estimation problem provide both worst-case performance bounds and explicit functions matching them, as our smooth (strongly) convex interpolation procedure is constructive. Our works build on those of Drori and Teboulle in [Math. Prog. 145 (1-2), 2014] who introduced and solved relaxations of the performance estimation problem for smooth convex functions. We apply our approach to different fixed-step first-order methods with several performance criteria, including objective function accuracy and gradient norm. We conjecture several numerically supported worst-case bounds on the performance of the fixed-step gradient, fast gradient and optimized gradient methods, both in the smooth convex and the smooth strongly convex cases, and deduce tight estimates of the optimal step size for the gradient method.

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