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NAG Library Manual

# NAG Library Routine DocumentS18CEF

Note:  before using this routine, please read the Users' Note for your implementation to check the interpretation of bold italicised terms and other implementation-dependent details.

## 1  Purpose

S18CEF returns a value of the scaled modified Bessel function ${e}^{-\left|x\right|}{I}_{0}\left(x\right)$ via the function name.

## 2  Specification

 FUNCTION S18CEF ( X, IFAIL)
 REAL (KIND=nag_wp) S18CEF
 INTEGER IFAIL REAL (KIND=nag_wp) X

## 3  Description

S18CEF evaluates an approximation to ${e}^{-\left|x\right|}{I}_{0}\left(x\right)$, where ${I}_{0}$ is a modified Bessel function of the first kind. The scaling factor ${e}^{-\left|x\right|}$ removes most of the variation in ${I}_{0}\left(x\right)$.
The routine uses the same Chebyshev expansions as S18AEF, which returns the unscaled value of ${I}_{0}\left(x\right)$.

## 4  References

Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications

## 5  Parameters

1:     X – REAL (KIND=nag_wp)Input
On entry: the argument $x$ of the function.
2:     IFAIL – INTEGERInput/Output
On entry: IFAIL must be set to $0$, $-1\text{​ or ​}1$. If you are unfamiliar with this parameter you should refer to Section 3.3 in the Essential Introduction for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value $-1\text{​ or ​}1$ is recommended. If the output of error messages is undesirable, then the value $1$ is recommended. Otherwise, if you are not familiar with this parameter, the recommended value is $0$. When the value $-\mathbf{1}\text{​ or ​}\mathbf{1}$ is used it is essential to test the value of IFAIL on exit.
On exit: ${\mathbf{IFAIL}}={\mathbf{0}}$ unless the routine detects an error or a warning has been flagged (see Section 6).

## 6  Error Indicators and Warnings

There are no actual failure exits from this routine. IFAIL is always set to zero. This parameter is included for compatibility with other routines in this chapter.

## 7  Accuracy

Relative errors in the argument are attenuated when propagated into the function value. When the accuracy of the argument is essentially limited by the machine precision, the accuracy of the function value will be similarly limited by at most a small multiple of the machine precision.

None.

## 9  Example

This example reads values of the argument $x$ from a file, evaluates the function at each value of $x$ and prints the results.

### 9.1  Program Text

Program Text (s18cefe.f90)

### 9.2  Program Data

Program Data (s18cefe.d)

### 9.3  Program Results

Program Results (s18cefe.r)