# 3.5.1.3.9 Bessel_i1

## Definition:

$i1 = bessel\_i1(x)$ evaluates an approximation to the modified Bessel function of the first kind,$I_1(x)$.The function is based on Chebyshev expansions.

For more information please review the s17afc function in the NAG document.

## Parameters:

x (input, double)
The argument x of the function.
i1 (output, double)
Approximation of to the modified Bessel function of the first kind.