Series of Functions

**shadow_2145** · June 19th 2008, 07:50 AM

(1)

(a) Show that $g(x)= \sum^{\infty}_{n=1}{cos\frac{(2^nx)}{2^n}}$ is continuous on all of $\mathbb{R}$

(b) Prove that $h(x)= \sum^{\infty}_{n=1}{\frac{x^n}{n^2}}$ is continuous on $[-1,1]$ .

(2) Observe that the series $f(x)=\sum^{\infty}_{n=1}{\frac{x^n}{n}}=x+\frac{x^ 2}{2}+\frac{x^3}{3}+\frac{x^4}{4}+...$ converges for every $x$ in the half-open interval $[0,1)$ but does not converge when $x=1$ . For a fixed $x_0 \in (0,1)$ , explain how we can still use the Weierstrass M-Test to prove that $f$ is continuous at $x_0$ .

(3) Let $h(x)=\sum^{\infty}_{n=1}{\frac{1}{x^2+n^2}}$

Show that $h$ is a continuous function defined on all of $\mathbb{R}$ .

Thanks!

**ThePerfectHacker** · June 19th 2008, 07:58 AM

You prove these problems uses the Weierstrass test. For example, in 1(a) we see that $\left| \tfrac{\cos 2^n x}{2^n} \right| \leq \tfrac{1}{2^n}$ and the series of $\tfrac{1}{2^n}$ converges. Thus, this series of fuctions converges uniformly. Since each function is continous we see that the limit (the infinite sum) is continous because the uniform limit of continous functions is continous.

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