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Или более полно
Dirichlet's theorem (sufficient convergence conditions for the Fourier series of a function)

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Note added at 22 mins (2004-08-17 08:56:51 GMT)
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Convergence of Fourier series
Now we are ready to prove the theorems giving the sufficient conditions for convergence of Fourier series . Note that there exist various sets of conditions that the given function must satisfy in order that it have a convergent Fourier series expansion. We shall prove one, maybe the simplest, theorem and state some others.

Theorem 3.2. (Dirichlet\'s Theorem) If f(x) is an absolutely integrable periodic function with period in the interval that has a finite number of points of discontinuity, then at any point where f(x) is differentiable its Fourier series expansion converges to the function f(x).
http://math.ut.ee/~toomas_l/harmonic_analysis/Fourier/node18...



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Note added at 8 hrs 40 mins (2004-08-17 17:14:38 GMT)
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В метематической энциклопедии эта теорема называется Теорема Дирихле о рядах Фурье и формулируется как условия сходимости ряда Фурье функции