### 8.28 `BESSEL_J1` — Bessel function of the first kind of order 1

Description:
  `BESSEL_J1(X)` computes the Bessel function of the first kind of order 1 of X. This function is available under the name `BESJ1` as a GNU extension.
Standard:
  Fortran 2008
Class:
  Elemental function
Syntax:
  `RESULT = BESSEL_J1(X)`
Arguments:

  X | The type shall be `REAL`, and it shall be scalar.

Return value:
  The return value is of type `REAL` and it lies in the range - 0.5818... \leq Bessel (0,x) \leq 0.5818 . It has the same kind as X.
Example:

                program test_besj1
                  real(8) :: x = 1.0_8
                  x = bessel_j1(x)
                end program test_besj1

Specific names:

  Name | Argument | Return type | Standard
  `DBESJ1(X)` | `REAL(8) X` | `REAL(8)` | GNU extension