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

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

Other double factorials: doubleFactorials, logDoubleFactorials

## Author

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

## Examples

DoubleFactorial (-4:0) # Return 1 if n < 2
#>  1 1 1 1 1
DoubleFactorial (2) # 2
#>  2
DoubleFactorial (5) # 1 * 3 * 5
#>  15
exp(LnDoubleFactorial.int (8)) # log(2 * 4 * 6 * 8)
#>  384
DoubleFactorial64(31)
#> integer64
#>  191898783962510625