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Coordinate descent : ウィキペディア英語版
Coordinate descent
Coordinate descent is a non-derivative optimization algorithm. To find a local minimum of a function, one does line search along one coordinate direction at the current point in each iteration. One uses different coordinate directions cyclically throughout the procedure.
==Description==

Coordinate descent is based on the idea that the minimization of a multivariable function F(\mathbf) can be achieved by minimizing it along one direction at a time, i.e., solving univariate (or at least much simpler) optimization problems in a loop. In the simplest case of ''cyclic coordinate descent'', one cyclically iterates through the directions, one at a time, minimizing the objective function with respect to each coordinate direction at a time. That is, in each iteration, for each variable of the problem in turn, the algorithm solves the optimization problem
:x^_i = \underset_1, \dots, x^_, y, x^k_, \dots, x^k_n).
Thus, one begins with an initial guess \mathbf^0 for a local minimum of F, and get a sequence
\mathbf^0, \mathbf^1, \mathbf^2, \dots iteratively.
By doing line search in each iteration, one automatically has
:F(\mathbf^0)\ge F(\mathbf^1)\ge F(\mathbf^2)\ge \dots.
It can be shown that this sequence has similar convergence properties as steepest descent. No improvement after one cycle of line search along coordinate directions implies a stationary point is reached.
This process is illustrated below.
File:coordinate descent.jpg

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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