BSI1

This function evaluates the modified Bessel function of the first kind of order one.

Function Return Value

BSI1 — Function value. (Output)

Required Arguments

X — Argument for which the function value is desired. (Input)

FORTRAN 90 Interface

Generic: BSI1 (X)

Specific: The specific interface names are S_BSI1 and D_BSI1.

FORTRAN 77 Interface

Single: BSI1 (X)

Double: The double precision function name is DBSI1.

Description

The Bessel function I1(x) is defined to be

 

The argument should not be so close to zero that I1(x) ≈ x/2 underflows, nor so large in absolute value that e∣x∣ and, therefore, I1(x) overflows.

Comments

Informational Error

 

Type	Code	Description
2	1	The function underflows because the absolute value of X is too small.

Example

In this example, I1(4.5) is computed and printed.

 

USE BSI1_INT

USE UMACH_INT

 

IMPLICIT NONE

! Declare variables

INTEGER NOUT

REAL VALUE, X

! Compute

X = 4.5

VALUE = BSI1(X)

! Print the results

CALL UMACH (2, NOUT)

WRITE (NOUT,99999) X, VALUE

99999 FORMAT (' BSI1(', F6.3, ') = ', F6.3)

END

Output

 

BSI1( 4.500) = 15.389