# Does the sum $\sum_{n=1}^{\infty}{a_nb_n}$ converge(fourier series coefficients)?

Let $f\in H(0,2\pi)$, with inner product $<f,g>=\int_0^{2\pi}{f(t)g(t)dt}$

$S_f=a_0 + \sum_{n=1}^{\infty}{a_ncos(nx)}+\sum_{n=1}^{\infty}{b_nsin(nx)}$, is the fourier series for f.

Where $a_0=\frac{1}{\sqrt{2\pi}}<f,cos(nx)>$, $a_n=\frac{1}{\sqrt{\pi}}<f,cos(nx)>$,$b_n=\frac{1}{\sqrt{\pi}}<f,sin(nx)>$

Does the sum $\sum_{n=1}^{\infty}{a_nb_n}$ converge?

Since $\{a_n\}_{n=0}^{\infty}\cup{\{b_n\}_{n=1}^{\infty}}$ is a complete orthonormal system, I was thinking if there meight be a function which has a fourier series coefficients $A_n=a_nb_n$, and then determine the convergence for the mentioned sum.

Or perhaps using a known test, since $\sum_{n=1}^{\infty}{b_n}$ is bounded, but then again, the sequence $a_n$ is not monotonic...

Any help would be appreciated =]

• The sequences $(a_n)_n$ and $(b_n)_n$ are both in $\ell^2$ (why?). Now use the Cauchy Schwarz inequality. – PhoemueX Mar 7 '15 at 20:26