Prove that $f(x)=\sum_{n=1}^\infty 1/2^n(\cos3^nx)$ is continuous but nowhere differentiable on $\mathbb{R}$.
I have proved the continuity part, but unable to do the second one. Thanks for any help.
Prove that $f(x)=\sum_{n=1}^\infty 1/2^n(\cos3^nx)$ is continuous but nowhere differentiable on $\mathbb{R}$.
I have proved the continuity part, but unable to do the second one. Thanks for any help.