Microsoft website has an introduction on GAMMA function.

I could download it and have a look, it's a ".xlsx" file and the formula is shown as `=_xlfn.GAMMA(2.5)`

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However, when I create a new excel ".xlsx" file, I can't use it, neither by write formula as `=_xlfn.GAMMA(2.5)" or "=GAMMA(2.5)`

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Did I missed something? Is there some library I shall register, like using Solver function?

Clarification

@dave-rook is right.

I'm using Excel 2007. The msdn page says "GAMMA" function applies to Excel 2013, so the problem is the version.