     Next: Equivalence of the order Up: 1: WHAT IS A Previous: Functions with discontinuities

## Fourier transformations

Very simple examples of Fourier transformations for functions with discontinuities are given Table 2.
 2|c|Table 2: Fourier Transformations e-| t | e-t.H(t) H(t) t e-t.H(t) It is very simple to determine the Fourier transforms of all the functions in Table 1 using these simple properties of the Fourier transformation: We can see from the Table 2 that the Fourier transforms of functions with discontinuities of order 0 behave at as .Meanwhile, in case of a discontinuity of order 1, behaves asymptotically as . Let us take, for instance, the third function from Table 1: . In this case, where Then and when  More generally, it is simple to prove that, if (i.e., is continuous), and if (i.e., is summable), then .Consequently, when , ,this convergence being not slower than .

We shall prove the following statement. Let f(t) be a right-sided function, and, let's suppose that, for all , (when ) in the same way as exp . Then: (2)

As f(t) is a right-sided function, we have: (3)
Let us integrate this integral by parts: (4) (5) (6)

Repeating this N times we obtain (7)

By definition of the order of discontinuity . From the condition concerning the behavior of f(k)(t) at , the integral on the righthand side of equation (7) converges to at . This means that the last equation is equivalent to equation (2). So the asymptotic behavior of the Fourier-transforms for high frequencies reflects the sharpness of the discontinuities of the (original) functions. Both notions (asymptotic behavior and discontinuities) are equivalent ones.     Next: Equivalence of the order Up: 1: WHAT IS A Previous: Functions with discontinuities
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1/13/1998