g05 Chapter Contents
g05 Chapter Introduction
NAG C Library Manual

# NAG Library Function Documentnag_rngs_chi_sq (g05lcc)

## 1  Purpose

nag_rngs_chi_sq (g05lcc) generates a vector of pseudorandom numbers taken from a ${\chi }^{2}$-distribution with $\nu$ degrees of freedom.

## 2  Specification

 #include #include
 void nag_rngs_chi_sq (Integer df, Integer n, double x[], Integer igen, Integer iseed[], NagError *fail)

## 3  Description

The distribution has PDF (probability density function)
 $fx= x12ν-1×e-x/2 212ν×12ν-1! if ​x>0; fx=0 otherwise.$
This is the same as a gamma distribution with parameters $\frac{1}{2}\nu$ and $2$.
One of the initialization functions nag_rngs_init_repeatable (g05kbc) (for a repeatable sequence if computed sequentially) or nag_rngs_init_nonrepeatable (g05kcc) (for a non-repeatable sequence) must be called prior to the first call to nag_rngs_chi_sq (g05lcc).

## 4  References

Kendall M G and Stuart A (1969) The Advanced Theory of Statistics (Volume 1) (3rd Edition) Griffin
Knuth D E (1981) The Art of Computer Programming (Volume 2) (2nd Edition) Addison–Wesley

## 5  Arguments

1:     dfIntegerInput
On entry: $\nu$, the number of degrees of freedom of the distribution.
Constraint: ${\mathbf{df}}\ge 1$.
2:     nIntegerInput
On entry: $n$, the number of pseudorandom numbers to be generated.
Constraint: ${\mathbf{n}}\ge 0$.
3:     x[n]doubleOutput
On exit: the $n$ pseudorandom numbers from the specified ${\chi }^{2}$-distribution.
4:     igenIntegerInput
On entry: must contain the identification number for the generator to be used to return a pseudorandom number and should remain unchanged following initialization by a prior call to nag_rngs_init_repeatable (g05kbc) or nag_rngs_init_nonrepeatable (g05kcc).
5:     iseed[$4$]IntegerCommunication Array
On entry: contains values which define the current state of the selected generator.
On exit: contains updated values defining the new state of the selected generator.
6:     failNagError *Input/Output
The NAG error argument (see Section 3.6 in the Essential Introduction).

## 6  Error Indicators and Warnings

On entry, argument $〈\mathit{\text{value}}〉$ had an illegal value.
NE_INT
On entry, ${\mathbf{df}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{df}}\ge 1$.
On entry, ${\mathbf{n}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{n}}\ge 0$.
NE_INTERNAL_ERROR
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.

## 7  Accuracy

Not applicable.

The time taken by nag_rngs_chi_sq (g05lcc) increases with $\nu$.

## 9  Example

This example prints five pseudorandom numbers from a ${\chi }^{2}$-distribution with five degrees of freedom, generated by a single call to nag_rngs_chi_sq (g05lcc), after initialization by nag_rngs_init_repeatable (g05kbc).

### 9.1  Program Text

Program Text (g05lcce.c)

None.

### 9.3  Program Results

Program Results (g05lcce.r)