# 3.5.1.3.12 Bessel_j1

## Definition:

$j1= bessel\_j1(x)$, evaluates an approximation to the Bessel function of the first kind $J_1(x)$ .

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.
j1 (output, double)
The approximation of the Bessel function of the first kind.