The trigonometric series \(\sum_{n=1}^\infty \frac{\sin nx}{\sqrt{n}}\) is not a Fourier series.
a-trigonometric-series-that-is-not-a-Fourier-series-Riemann-image
The trigonometric series \(\sum_{n=1}^\infty \frac{\sin nx}{\sqrt{n}}\) is not a Fourier series.