[多變] 請益
先定義這裡提到的O(h).....little oh of h
there exists a vector y s.t. [f(x+h)-f(x)]-y˙h=O(h)----(*)
其中h=(h1,h2,.....,hn),h is a vector near 原點
the vector y which satisfy (*) is called the gradient of f at x
and denote it by ▽f(x)
<試證明>if 存在 vector y1 and y2 s.t. [f(x+h)-f(x)]-y1˙h=O(h)
and [f(x+h)-f(x)]-y2˙h=O(h)
then y1=y2
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