烏籠...

今日上 math 323 tutorial, 而且今日功課 dead line, tutorial 完左照舊又係同人吹吹 topology d 野, 跟住發覺我將所有功課裏面有關喺 topological space $  (X,\mathcal{T})$ 上,  collection of limit point of $  A$ 嘅 definition 寫左做 \[
x\in A' \iff \forall U\in N_x\triangleq \{V\in \mathcal{T}:x\in V\},( U-\{x\})\cap A\neq \emptyset.
\] 咁我呢 D 乖學生黎, 有錯梗係即改, 但改好哂之後就喺到諗, $  ( U-\{x\})\cap A$ 同 $  U\cap (A-\{x\})$ 好似冇分別喎......=.=.

Differentiation under the integral sign

http://en.wikipedia.org/wiki/Differentiation_under_the_integral_sign

原來有好多人都唔知呢條式嘅存在...，anyway 我都 state 左條 formula 同 condition 俾班同學聽，唔知佢地會唔會唔識證 (正常知條 formula 嘅詁 year 1 知識就夠哂數=.=)。