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

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

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