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Godofgamers
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Joined: 31 Mar 2005
Location: South Africa

PostPosted: Wed Oct 15, 2008 3:31 pm    Post subject:  Math Time Reply with quote  Mark this post and the followings unread

I have a question that has yet to be clearly answered:

Does the factorial function have an inverse, and if so, what is it?

So far I've found some pretty obfuscating stuff:

http://www.physicsforums.com/showthread.php?t=205761

http://www.halfbakery.com/idea/Inverse_20Factorial_20_27_3f_27

http://www.mathlinks.ro/Forum/viewtopic.php?p=74635#74635

It seems that the answer lies within the inverse regularised gamma function, but nowhere that I've checked gives me the actual definition of the function.

Thus far I've found two other (although not terribly useful) possibilities to define the inverse factorial.

1. Divide by consecutive positive integers until you get a non-integer result, or 1. the number you divided by to get one will be the answer.

2. x = y! (Not very useful)

Does anyone have any ideas or info to contribute?

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Clazzy
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Joined: 26 Nov 2002
Location: Algae Colony On Mars

PostPosted: Wed Oct 15, 2008 4:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Factorials aren't like squares and square roots, not every real number can be used in a factorial. The big killer to this is that only a certain subset of real numbers can even have factorials. You can divide each number into its prime factors but that doesn't really get you what you want.
From what I can see of the gamma function (I'm afraid I'm not a mathematician so I can't give a definite answer) it is a generalised form of factorials because it works for all real and complex numbers. However, Γ(4) isn't 4!, it's 3! which makes me think it could possibly be a slightly different function than just merely factorials. If that's the case, the inverse gamma function (which I can't find anything on either) isn't actually giving the factorial. I suggest you find a mathematician and bug them, they're probably more useful and a forum mostly filled with 12-16 year olds and a few university undergraduates plus random others.

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Godofgamers
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Joined: 31 Mar 2005
Location: South Africa

PostPosted: Wed Oct 15, 2008 4:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, that's what I thought. Although, the gamma function of x+1 is equivalent to the factorial function (were it extended to the set of all real numbers).

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