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Subderivative : ウィキペディア英語版
Subderivative

In mathematics, the subderivative, subgradient, and subdifferential generalize the derivative to functions which are not differentiable. The subdifferential of a function is set-valued. Subderivatives arise in convex analysis, the study of convex functions, often in connection to convex optimization.
Let ''f'':''I''→R be a real-valued convex function defined on an open interval of the real line. Such a function need not be differentiable at all points: For example, the absolute value function ''f''(''x'')=|''x''| is nondifferentiable when ''x''=0. However, as seen in the picture on the right, for any ''x''0 in the domain of the function one can draw a line which goes through the point (''x''0, ''f''(''x''0)) and which is everywhere either touching or below the graph of ''f''. The slope of such a line is called a ''subderivative'' (because the line is under the graph of ''f'').
==Definition==
Rigorously, a ''subderivative'' of a function ''f'':''I''→R at a point ''x''0 in the open interval ''I'' is a real number ''c'' such that
:f(x)-f(x_0)\ge c(x-x_0)
for all ''x'' in ''I''. One may show that the set of subderivatives at ''x''0 for a convex function is a nonempty closed interval (''b'' ), where ''a'' and ''b'' are the one-sided limits
:a=\lim_\frac
:b=\lim_\frac
which are guaranteed to exist and satisfy ''a'' ≤ ''b''.
The set (''b'' ) of all subderivatives is called the subdifferential of the function ''f'' at ''x''0. If ''f'' is convex and its subdifferential at x_0 contains exactly one subderivative, then ''f'' is differentiable at x_0.〔R. T. Rockafellar ''Convex analysis'' 1970. Theorem 25.1, p.242〕

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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