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Eulerian functions. Classical orthogonal polynomials

The content of this paragraph consists of reminders of fundamental results, used in the following.

Special functions are, in general, constructed from the fundamental procedures of analysis: limit crossing in a finite sum of elements where the number of elements grows indefinitely, which gives series, and limit crossing in a finite product of elements, which gives infinite products. The integral is defined as the limit of a finite sum, but this is a more complex process; Riemann integral theory is assumed.