Double Factorial

[Jason Merrill]

I couldn't find a double factorial function in sage. That is,

n!! == n*(n - 2)*(n - 4)...

Does Sage have a double factorial somewhere that I'm missing. If not,
could it?

Regards,

JM
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[William Stein]

Post by Jason Merrill
I couldn't find a double factorial function in sage. That is,
n!! == n*(n - 2)*(n - 4)...
Does Sage have a double factorial somewhere that I'm missing. If not,
could it?

I had to write this for Pynac. Here's the 1-liner that does it...

def doublefactorial(x):
n = Integer(x)
if n < -1:
raise ValueError, "argument must be >= -1"
return prod([n - 2*i for i in range(n//2)])

sage: doublefactorial(20)
3715891200

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[Robert Bradshaw]

Post by William Stein

Post by Jason Merrill
I couldn't find a double factorial function in sage. That is,
n!! == n*(n - 2)*(n - 4)...
Does Sage have a double factorial somewhere that I'm missing. If not,
could it?

I had to write this for Pynac. Here's the 1-liner that does it...
n = Integer(x)
raise ValueError, "argument must be >= -1"
return prod([n - 2*i for i in range(n//2)])
sage: doublefactorial(20)
3715891200

Yes, Integers have a (highly optimized) multifactorial method.

sage: a = 20

sage: a.multifactorial(2)
3715891200

- Robert


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[Jason Merrill]

Post by Robert Bradshaw

Does Sage have a double factorial somewhere that I'm missing.  If not,
could it?

Yes, Integers have a (highly optimized) multifactorial method.
sage: a = 20
sage: a.multifactorial(2)
  3715891200
- Robert

Perfect. Thanks.

JM
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