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The Fourier series of a complex-valued P-periodic function (), integrable over the interval [,] on the real line, is defined as a trigonometric series of the form =, such that the Fourier coefficients are complex numbers defined by the integral [14] [15] = .
An Elementary Treatise on Fourier's Series: And Spherical, Cylindrical, and Ellipsoidal Harmonics, with Applications to Problems in Mathematical Physics (2 ed.). Ginn. p. 30. Carslaw, Horatio Scott (1921). "Chapter 7: Fourier's Series". Introduction to the Theory of Fourier's Series and Integrals, Volume 1 (2 ed.). Macmillan and Company. p. 196.
Consider f an integrable function on the interval [0, 2π].For such an f the Fourier coefficients ^ are defined by the formula ^ = (),. It is common to describe the connection between f and its Fourier series by
If () is a periodic function, with period , that has a convergent Fourier series, then: ^ = = (), where are the Fourier series coefficients of , and is the Dirac delta function. In other words, the Fourier transform is a Dirac comb function whose teeth are multiplied by the Fourier series coefficients.
These are called Fourier series coefficients. The term Fourier series actually refers to the inverse Fourier transform, which is a sum of sinusoids at discrete frequencies, weighted by the Fourier series coefficients. When the non-zero portion of the input function has finite duration, the Fourier transform is continuous and finite-valued.
In modern times, variants of the discrete Fourier transform were used by Alexis Clairaut in 1754 to compute an orbit, [16] which has been described as the first formula for the DFT, [17] and in 1759 by Joseph Louis Lagrange, in computing the coefficients of a trigonometric series for a vibrating string. [17]
The result of the series is also a function of the discrete variable, i.e. a discrete sequence. A Fourier series, by nature, has a discrete set of components with a discrete set of coefficients, also a discrete sequence. So a DFS is a representation of one sequence in terms of another sequence.
In mathematics, the Poisson summation formula is an equation that relates the Fourier series coefficients of the periodic summation of a function to values of the function's continuous Fourier transform. Consequently, the periodic summation of a function is completely defined by discrete samples of the original function's Fourier transform.