R: Double factorial

DoubleFactorial {TreeTools} | R Documentation

## Double factorial

### Description

Calculate the double factorial of a number, or its logarithm.

### Usage

    DoubleFactorial(n)

    DoubleFactorial64(n)

    LnDoubleFactorial(n)

    Log2DoubleFactorial(n)

    LogDoubleFactorial(n)

    LnDoubleFactorial.int(n)

    LogDoubleFactorial.int(n)

### Arguments

`n` | Vector of integers.

### Value

Returns the double factorial, n * (n - 2) * (n - 4) * (n - 6) * ...

### Functions

- `DoubleFactorial64`: Returns the exact double factorial as a 64-bit `integer64`, for `n` < 34.

- `LnDoubleFactorial`: Returns the logarithm of the double factorial.

- `Log2DoubleFactorial`: Returns the logarithm of the double factorial.

- `LnDoubleFactorial.int`: Slightly faster, when x is known to be length one and below 50001

### Author(s)

Martin R. Smith (martin.smith@durham.ac.uk)

### See Also

Other double factorials: `doubleFactorials`, `logDoubleFactorials`

### Examples

    DoubleFactorial (-4:0) # Return 1 if n < 2
    DoubleFactorial (2) # 2
    DoubleFactorial (5) # 1 * 3 * 5
    exp(LnDoubleFactorial.int (8)) # log(2 * 4 * 6 * 8)
    DoubleFactorial64(31)

[Package TreeTools version 1.4.4 Index]