Chapter 6: Bessel Functions

BSI0

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

Function Return Value

BSI0 — Function value.   (Output)

Required Arguments

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

FORTRAN 90 Interface

Generic:                              BSI0 (X)

Specific:                             The specific interface names are S_BSI0 and D_BSI0.

FORTRAN 77 Interface

Single:                                BSI0 (X)

Double:                              The double precision function name is DBSI0.

Description

The Bessel function I0(x) is defined to be

The absolute value of the argument x must not be so large that e|x| overflows.

Figure 6- 3  Plot of I0(x) and I1(x)

Example

In this example, I0 (4.5) is computed and printed.

 

      USE BSI0_INT

      USE UMACH_INT

 

      IMPLICIT   NONE

!                                 Declare variables

      INTEGER    NOUT

      REAL       VALUE, X

!                                 Compute

      X     = 4.5

      VALUE = BSI0(X)

!                                 Print the results

      CALL UMACH (2, NOUT)

      WRITE (NOUT,99999) X, VALUE

99999 FORMAT (' BSI0(', F6.3, ') = ', F6.3)

      END

Output

 

BSI0( 4.500) = 17.481



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