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NAG C Library Manual

# NAG Library Function Documentnag_arccosh (s11acc)

## 1  Purpose

nag_arccosh (s11acc) returns the value of the inverse hyperbolic cosine, $\mathrm{arccosh}x$. The result is in the principal positive branch.

## 2  Specification

 #include #include
 double nag_arccosh (double x, NagError *fail)

## 3  Description

nag_arccosh (s11acc) calculates an approximate value for the inverse hyperbolic cosine, $\mathrm{arccosh}x$. It is based on the relation
 $arccosh⁡x = ln x + x 2 - 1 .$
This form is used directly for $1, where $k=n/2+1$, and the machine uses approximately $n$ decimal place arithmetic.
For $x\ge {10}^{k},\sqrt{{x}^{2}-1}$ is equal to $\sqrt{x}$ to within the accuracy of the machine and hence we can guard against premature overflow and, without loss of accuracy, calculate
 $arccosh⁡x = ln⁡2 + ln⁡x .$

## 4  References

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

## 5  Arguments

1:     xdoubleInput
On entry: the argument $x$ of the function.
Constraint: ${\mathbf{x}}\ge 1.0$.
2:     failNagError *Input/Output
The NAG error argument (see Section 3.6 in the Essential Introduction).

## 6  Error Indicators and Warnings

NE_REAL_ARG_LT
On entry, x must not be less than 1.0: ${\mathbf{x}}=〈\mathit{\text{value}}〉$.
$\mathrm{arccosh}x$ is not defined and the result returned is zero.

## 7  Accuracy

If $\delta$ and $\epsilon$ are the relative errors in the argument and result respectively, then in principle
 $ε ≃ x x 2 - 1 arccosh⁡x δ .$
That is the relative error in the argument is amplified by a factor at least
 $x x 2 - 1 arccosh⁡x$
in the result. The equality should apply if $\delta$ is greater than the machine precision ($\delta$ due to data error etc.), but if $\delta$ is simply a result of round-off in the machine representation, it is possible that an extra figure may be lost in internal calculation and round-off.
It should be noted that for $x>2$ the factor is always less than 1.0. For large $x$ we have the absolute error $E$ in the result, in principle, given by
 $E ∼ δ .$
This means that eventually accuracy is limited by machine precision. More significantly for $x$ close to 1, $x-1\sim \delta$, the above analysis becomes inapplicable due to the fact that both function and argument are bounded, $x\ge 1$, $\mathrm{arccosh}x\ge 0$. In this region we have
 $E ∼ δ .$
That is, there will be approximately half as many decimal places correct in the result as there were correct figures in the argument.

None.

## 9  Example

The following program 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 (s11acce.c)

### 9.2  Program Data

Program Data (s11acce.d)

### 9.3  Program Results

Program Results (s11acce.r)