上海交通大学2012年数学分析试题
证明
(1)、
用分部积分法得
$\displaystyle I_n=\int_{0}^{1}f(t^n)dt=tf(t^n)|_0^1-\int_{0}^{1}nt^{n}f'(t^n)dt=f(0)-\int_{0}^{1}nt^{n}f'(t^n)dt,$
而
$\displaystyle \lim_{n \to \infty }\int_{0}^{1}nt^{n}f'(t^n)dt=\lim_{n \to \infty }\int_{0}^{1-\delta }nt^{n}f'(t^n)dt+\lim_{n \to \infty }\int_{1-\delta }^{1}nt^{n}f'(t^n)dt,(0< \delta < 1)$
由已知
$\because f(x)\in C[0,1],$
$\therefore |f'(x)|\leq M,$
又
$\displaystyle \because nt^{n}f'(t^n)\rightarrow 0,(n \to \infty ,t\in[0,1))$
所以,上述极限第一项被积函数一致收敛,从而积分和极限可变换次序
$\displaystyle \therefore \lim_{n \to \infty }\int_{0}^{1-\delta }nt^{n}f'(t^n)dt=\int_{0}^{1-\delta }\lim_{n \to \infty }nt^{n}f'(t^n)dt=0.$
再由导函数的有界性,得
$\displaystyle \Rightarrow \lim_{n \to \infty }\int_{1-\delta }^{1}nt^{n}f'(t^n)dt\leq \lim_{n \to \infty }M\int_{1-\delta }^{1}nt^{n}dt=\lim_{n \to \infty }M\frac{n}{n+1}[1-(1-\delta )^{n+1}]=0.$
从而
$\displaystyle \Rightarrow \lim_{n \to \infty }I_n=f(0)-\lim_{n \to \infty }\int_{0}^{1}nt^{n}f'(t^n)dt=f(0).$
(2)、