In q-analog theory, the q-gamma function, or basic gamma function, is a generalization of the ordinary Gamma function closely related to the double gamma function. It was introduced by Jackson (1905). It is given by
where (·;·)∞ is the infinite q-Pochhammer symbol. It satisfies the functional equation
For non-negative integers n,
where q! is the q-factorial function. Alternatively, this can be taken as an extension of the q-factorial function to the real number system.
The relation to the ordinary gamma function is made explicit in the limit
A q-analogue of Stirling's formula is given by
A q-analogue of the multiplication formula is given by
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