For alternative uses of the exclamation point symbol, please see double factorial for expressions of the form 4!! or subfactorial for expressions of the form !3 .
The notation is read " factorial". Alternatively, one could think of the product as being in the opposite order:
As a consequence of the empty product,
As a concrete example:
The factorial function can also be seen as a specific case of the gamma function (), which extends the factorial to the complex plane (excluding the non-positive integers). In particular, for all values for which the factorial is defined: