# NAG FL Interfaces01baf (log_​shifted)

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## 1Purpose

s01baf returns a value of the shifted logarithmic function, $\mathrm{ln}\left(1+x\right)$, via the function name.

## 2Specification

Fortran Interface
 Function s01baf ( x,
 Real (Kind=nag_wp) :: s01baf Integer, Intent (Inout) :: ifail Real (Kind=nag_wp), Intent (In) :: x
#include <nag.h>
 double s01baf_ (const double *x, Integer *ifail)
The routine may be called by the names s01baf or nagf_specfun_log_shifted.

## 3Description

s01baf computes values of $\mathrm{ln}\left(1+x\right)$, retaining full relative precision even when $|x|$ is small. The routine is based on the Chebyshev expansion
 $ln⁡1+p2+2px¯ 1+p2-2px¯ =4∑k=0∞p2k+1 2k+1 T2k+1(x¯).$
Setting $\overline{x}=\frac{x\left(1+{p}^{2}\right)}{2p\left(x+2\right)}$, and choosing $p=\frac{q-1}{q+1}$, $q=\sqrt{2}$ the expansion is valid in the domain $x\in \left[\frac{1}{\sqrt{2}}-1,\sqrt{2}-1\right]$.
Outside this domain, $\mathrm{ln}\left(1+x\right)$ is computed by the standard logarithmic function.
Lyusternik L A, Chervonenkis O A and Yanpolskii A R (1965) Handbook for Computing Elementary Functions p. 57 Pergamon Press

## 5Arguments

1: $\mathbf{x}$Real (Kind=nag_wp) Input
On entry: the argument $x$ of the function.
Constraint: ${\mathbf{x}}>-1.0$.
2: $\mathbf{ifail}$Integer Input/Output
On entry: ifail must be set to $0$, $-1$ or $1$ to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of $0$ causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of $-1$ means that an error message is printed while a value of $1$ means that it is not.
If halting is not appropriate, the value $-1$ or $1$ is recommended. If message printing is undesirable, then the value $1$ is recommended. Otherwise, the value $0$ is recommended. When the value $-\mathbf{1}$ 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).

## 6Error Indicators and Warnings

If on entry ${\mathbf{ifail}}=0$ or $-1$, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
${\mathbf{ifail}}=1$
On entry, ${\mathbf{x}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{x}}>-1.0$.
${\mathbf{ifail}}=-99$
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

## 7Accuracy

The returned result should be accurate almost to machine precision, with a limit of about $20$ significant figures due to the precision of internal constants. Note however, that if $x$ lies very close to $-1.0$ and is not exact (for example if $x$ is the result of some previous computation and has been rounded), then precision will be lost in the computation of $1+x$, and hence $\mathrm{ln}\left(1+x\right)$, in s01baf.

## 8Parallelism and Performance

s01baf is not threaded in any implementation.

Empirical tests show that the time taken for a call of s01baf usually lies between about $1.25$ and $2.5$ times the time for a call to the standard logarithm function.

## 10Example

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

### 10.1Program Text

Program Text (s01bafe.f90)

### 10.2Program Data

Program Data (s01bafe.d)

### 10.3Program Results

Program Results (s01bafe.r)