let {\(\displaystyle v_k\):k=1,2,3,....} be an orthonormal sequence in C[0,2pi], given by \(\displaystyle v_k(x)=sinkx/pi\).

Give an example of a function u such \(\displaystyle u\neq\sum_{k=1}<u,v_k>v_k\) sum from k=1 to infinity.

thanks for any help.