## Interface

erlang(n:int, r:real) : real

where `n>=1`

and `r>0.0`

. Returns a drawing from an `n`

-Erlang

distribution with intensity `r`

.

A drawing from an `n`

-Erlang distribution can be derived by addition of

`n`

drawings from a exponential distribution.

Raises `Erlang`

exception, if `n`

<1 or `r<=0.0`

.

## Characteristics

- Mean:
`n/r`

- Variance:
`n/r^2`

Density functions for `n`

-Erlang distributions:

## Example

erlang(100,50.0)

A shop gives each `100`

th customer a present. The arrival time

between customers is exponentially distributed with intensity `r=50.0`

per hour. The sums of the arrival times between customers is Erlang

distributed with parameters `n=100`

and `r=50.0`

.

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