Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Possible Duplicate:
Is there an Inverse Gamma $\Gamma^{-1} (z) $ function?

Is there any way I can 'undo' the factorial operation? JUst like you can do squares and square roots, can you do factorials and factorial roots (for lack of a better term)?

Here is an example: 5! = 120. Is there a way I can work out the number that must be factorialed (??) to give the answer 120?

share|improve this question

marked as duplicate by J. M., lhf, hardmath, Did, Asaf Karagila Jul 17 '12 at 11:54

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

3  
Only numerically. The inverse of the gamma function (factorial) does not have a simple, elementary expression. –  J. M. Jul 17 '12 at 10:52
2  
Possibly related topic. –  johnny Jul 17 '12 at 10:52
    

1 Answer 1

You can just divide the "answer" by consecutive positive integers, and when the result is 1, the last number you divided by is the number that the "answer" is factorial of. For example: 120 / 2 = 60, 60 / 3 = 20, 20 / 4 = 5, 5 / 5 = 1, so the number that 120 is the factorial of is 5.

share|improve this answer
3  
This would probably be the simplest and quickest way to do it (maybe not by hand for large numbers). If at some stage, you divide by $k$ exactly, and at the next step you cannot divide by $k+1$ exactly, then, clearly, you do not have a factorial number. –  digital-Ink Jul 17 '12 at 11:43

Not the answer you're looking for? Browse other questions tagged or ask your own question.