This function calculates the first kind Bessel function
of order 0, for any REAL X. The polynomial approximation by
series of Chebyshev polynomials is used for 0<X<8 and 0<8/X<1. References:
M.Abramowitz, I.A.Stegun, Handbook of Mathematical Functions, 1965. C.W.Clenshaw, Chebyshev Series for Mathematical Functions, Mathematical Tables, Vol. 5, 1962.

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