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Chapter Contents
Chapter Introduction
NAG Toolbox

# NAG Toolbox: nag_rand_dist_students_t (g05sn)

## Purpose

nag_rand_dist_students_t (g05sn) generates a vector of pseudorandom numbers taken from a Student's $t$-distribution with $\nu$ degrees of freedom.

## Syntax

[state, x, ifail] = g05sn(n, df, state)
[state, x, ifail] = nag_rand_dist_students_t(n, df, state)

## Description

The distribution has PDF (probability density function)
 $fx= ν-12 ! 12ν-1!πν 1+x2ν 12ν+1 .$
nag_rand_dist_students_t (g05sn) calculates the values
 $yiνzi, i= 1,…,n$
where the ${y}_{i}$ are generated by nag_rand_dist_normal (g05sk) from a Normal distribution with mean $0$ and variance $1.0$, and the ${z}_{i}$ are generated by nag_rand_dist_gamma (g05sj) from a gamma distribution with parameters $\frac{1}{2}\nu$ and $2$ (i.e., from a ${\chi }^{2}$-distribution with $\nu$ degrees of freedom).
One of the initialization functions nag_rand_init_repeat (g05kf) (for a repeatable sequence if computed sequentially) or nag_rand_init_nonrepeat (g05kg) (for a non-repeatable sequence) must be called prior to the first call to nag_rand_dist_students_t (g05sn).

## References

Knuth D E (1981) The Art of Computer Programming (Volume 2) (2nd Edition) Addison–Wesley

## Parameters

### Compulsory Input Parameters

1:     $\mathrm{n}$int64int32nag_int scalar
$n$, the number of pseudorandom numbers to be generated.
Constraint: ${\mathbf{n}}\ge 0$.
2:     $\mathrm{df}$int64int32nag_int scalar
$\nu$, the number of degrees of freedom of the distribution.
Constraint: ${\mathbf{df}}\ge 1$.
3:     $\mathrm{state}\left(:\right)$int64int32nag_int array
Note: the actual argument supplied must be the array state supplied to the initialization routines nag_rand_init_repeat (g05kf) or nag_rand_init_nonrepeat (g05kg).
Contains information on the selected base generator and its current state.

None.

### Output Parameters

1:     $\mathrm{state}\left(:\right)$int64int32nag_int array
Contains updated information on the state of the generator.
2:     $\mathrm{x}\left({\mathbf{n}}\right)$ – double array
The $n$ pseudorandom numbers from the specified Student's $t$-distribution.
3:     $\mathrm{ifail}$int64int32nag_int scalar
${\mathbf{ifail}}={\mathbf{0}}$ unless the function detects an error (see Error Indicators and Warnings).

## Error Indicators and Warnings

Errors or warnings detected by the function:
${\mathbf{ifail}}=1$
Constraint: ${\mathbf{n}}\ge 0$.
${\mathbf{ifail}}=2$
Constraint: ${\mathbf{df}}\ge 1$.
${\mathbf{ifail}}=3$
On entry, state vector has been corrupted or not initialized.
${\mathbf{ifail}}=-99$
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.

## Accuracy

Not applicable.

The time taken by nag_rand_dist_students_t (g05sn) increases with $\nu$.

## Example

This example prints five pseudorandom numbers from a Student's $t$-distribution with five degrees of freedom, generated by a single call to nag_rand_dist_students_t (g05sn), after initialization by nag_rand_init_repeat (g05kf).
```function g05sn_example

fprintf('g05sn example results\n\n');

% Initialize the base generator to a repeatable sequence
seed  = [int64(1762543)];
genid = int64(1);
subid = int64(1);
[state, ifail] = g05kf( ...
genid, subid, seed);

% Number of variates
n = int64(5);

% Parameters
df = int64(5);

% Generate variates from a Student t distribution
[state, x, ifail] = g05sn( ...
n, df, state);

disp('Variates');
disp(x);

```
```g05sn example results

Variates
0.3849
-0.9461
-2.2814
0.1127
0.5272

```