CHI

This function evaluates the hyperbolic cosine integral.

Function Return Value

CHI — Function value. (Output)

Required Arguments

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

FORTRAN 90 Interface

Generic: CHI (X)

Specific: The specific interface names are S_CHI and D_CHI.

FORTRAN 77 Interface

Single: CHI (X)

Double: The double precision function name is DCHI.

Description

The hyperbolic cosine integral, Chi(x), is defined to be

 

where  ≈ 0.57721566 is Euler’s constant.

The argument x must be large enough that e−x/x does not underflow, and x must be small enough that ex does not overflow.

Comments

When X is negative, the principal value is used.

Example

In this example, Chi(2.5) is computed and printed.

 

USE CHI_INT

USE UMACH_INT

 

IMPLICIT NONE

! Declare variables

INTEGER NOUT

REAL VALUE, X

! Compute

X = 2.5

VALUE = CHI(X)

! Print the results

CALL UMACH (2, NOUT)

WRITE (NOUT,99999) X, VALUE

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

END

Output

 

CHI(2.500) = 3.524