Bessel's inequality in Fourier series

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bessel inequality

In Bessel's inquality, how does the series of sum of the squares of Fourier coefficients converges ? Is there any particular result for the convergence of series that they have used ?

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The partial sums of a serise of non-negative numbers is an increasing sequence. If it is bounded then it is convergent. In this case the bound is $\frac 1 {\pi} \int_{-\pi}^{\pi} f^{2}(t)dt$.