[Math] Uniform convergence of $f_n(x)=\frac{1}{1+nx}$ on $(0,1)$

real-analysissequences-and-seriesuniform-convergence

Consider the sequence of functions $f_n(x)=\frac{1}{1+nx}$ for $x\in (0,1)$. Then

  1. $f_n (x) → 0$ pointwise but not uniformly on $(0,1)$.
  2. $f_n (x) → 0$ uniformly on $(0,1)$.
  3. $\int_{0}^{1} f_n (x) dx → 0$ as $n → ∞$.
  4. $f'_n(1/n)\to 0$ as $n\to \infty$.

I have shown that $3$ and $4$ are false. As $\lim_{n\to \infty} \int_{0}^{1}f_n(x)dx=1$ and $f'_n(1/n)=-\frac{n}{4}$. But I am confused about the convergence part. I know $f_n(x)$ is not uniformly convergent on $[0,1]$, so can I just remove the end points and claim that it is also not uniformly convergent on $(0,1)$ as well. It does not feels rigorous. How can I do this. Please help me out.

Best Answer

Let $\epsilon=\frac{1}{2}$, $n_k = k+1$, and $x_k=\frac{1}{k+1}$, then \begin{align} |f_{n_k}(x_k)-f(x_k)| &= \left|\frac{1}{1+(k+1)\cdot\frac{1}{k+1}}\right|\\ &=\frac{1}{2}\\ &\ge \epsilon, \end{align} so $(f_n)$ does not uniformly converge.

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