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发帖数: 2911 | 1 This is an elementary technique.
f(x+h,y) - f(x,y) = \int_0^1 d/dt {f(x+th,y)} dt
= \int_0^1 df/dx(x+th,y)*h dt
= \int_0^1 \int_0^1 d/ds {df/dx(x+th,sy)*h} ds dt
= \int_0^1 \int_0^1 d^2 f/dxdy (x+th,sy)*h*y ds dt
Take norm on both sides. |
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