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The German astronomer F. W. Bessel (1784â1846) first achieved fame by computing the orbit of Halley's comet. In addition to many other accomplishments in connection with his studies of planetary motion, he is credited with deriving the differential equation bearing his name and carrying out the first systematic study of the general properties of its solutions (now called Bessel functions) in his famous 1824 memoir. Nonetheless, Bessel functions were first discovered in 1732 by D. Bernoulli (1700â1782), who provided a series solution (representing a Bessel function) for the oscillatory displacements of a heavy hanging chain (see Sec. 6.7.1). Euler later developed a series similar to that of Bernoulli, which was also a Bessel function, and Bessel's equation appeared in a 1764 article by Euler dealing with the vibrations of a circular drumhead. J. Fourier (1768â1836) also used Bessel functions in his classical treatise on heat in 1822, but it was Bessel who first recognized their special properties.
Bessel functions are closely associated with problems possessing circular or cylindrical symmetry. For example, they arise in the study of free vibrations of a circular membrane and in finding the temperature distribution in a circular cylinder. They also occur in electromagnetic theory and numerous other areas of physics and engineering. In fact, Bessel functions occur so frequently in practice that they are undoubtedly the most important functions beyond the elementary ones.
Because of their close association with cylindrical domains, the solutions of Bessel's equation are also called cylinder functions.
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