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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?

  • 3
    Only numerically. The inverse of the gamma function (factorial) does not have a simple, elementary expression.2012-07-17
  • 2
    Possibly [related](http://mathoverflow.net/questions/12828/inverse-gamma-function) topic.2012-07-17
  • 0
    possible duplicate of [Is there an Inverse Gamma $\Gamma^{-1} (z) $ function?](http://math.stackexchange.com/questions/18362) and [Is there a way to solve for an unknown in a factorial?](http://math.stackexchange.com/questions/61755)2012-07-17

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