# 3.5.1.3.44 J1

## Description

This function returns the first order Bessel function:

J1(x)

The formula for the equation is:

$J_1(x)=\sum_{k=0}^{\infty} \frac{(-1)^{k}(x/2)^{2k+1}}{k!\Gamma(k+2)}$

See the gammaln(x) function for the definition of $\Gamma$.

## Syntax

double J1(double x)

## Parameter

x

is the double at which you want to calculate the first order Bessel function.

## Return

Returns the first order Bessel function at x.

## Example

aa = j1(5);
aa =; //-0.32757913759147