bessel_i1_realreturns a value for the modified Bessel function .
For full information please refer to the NAG Library document for s18af
The argument of the function.
The value of the function at .
- (errno )
On entry, .
is too large and the function returns the approximate value of at the nearest valid argument.
bessel_i1_realevaluates an approximation to the modified Bessel function of the first kind .
Note: , so the approximation need only consider .
The function is based on three Chebyshev expansions:
For small , . This approximation is used when is sufficiently small for the result to be correct to machine precision.
For large , the function must fail because cannot be represented without overflow.
NIST Digital Library of Mathematical Functions