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(Odd) harmonics of a 1000 Hz square wave Graph showing the first 3 terms of the Fourier series of a square wave Using Fourier expansion with cycle frequency f over time t , an ideal square wave with an amplitude of 1 can be represented as an infinite sum of sinusoidal waves: x ( t ) = 4 π ∑ k = 1 ∞ sin ( 2 π ( 2 k − 1 ) f t ) 2 k ...
A square wave (represented as the blue dot) is approximated by its sixth partial sum (represented as the purple dot), formed by summing the first six terms (represented as arrows) of the square wave's Fourier series. Each arrow starts at the vertical sum of all the arrows to its left (i.e. the previous partial sum).
A widespread anecdote says that when the Fourier coefficients for a square wave were input to the machine, the graph would oscillate at the discontinuities, and that because it was a physical device subject to manufacturing flaws, Michelson was convinced that the overshoot was caused by errors in the machine.
Left: A continuous function (top) and its Fourier transform (bottom). Center-left: Periodic summation of the original function (top). Fourier transform (bottom) is zero except at discrete points. The inverse transform is a sum of sinusoids called Fourier series. Center-right: Original function is discretized (multiplied by a Dirac comb) (top).
The Fourier series for the identity function suffers from the Gibbs phenomenon near the ends of the periodic interval. Every Fourier series gives an example of a trigonometric series. Let the function () = on [,] be extended periodically (see sawtooth wave). Then its Fourier coefficients are:
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.
A generalized Fourier series is the expansion of a square integrable function into a sum of square integrable orthogonal basis functions. The standard Fourier series uses an orthonormal basis of trigonometric functions , and the series expansion is applied to periodic functions.
Since the optical transfer function is the Fourier transform of the point spread function, and the point spread function is the square absolute of the inverse Fourier transformed pupil function, the optical transfer function can also be calculated directly from the pupil function.