L10_A
The existence of partial derivative of functions of several variables fails to guarentee continuity (多變數函數偏微分的存在性並無法保證其函數連續)
二階偏微分的順序

L10_B
Ch.16 Gradients;Extreme values, differential.
16.1 Differentiability and Gradient. (可微分性)
little-o(x)