by definition. The q-Pochhammer symbol is a major building block in the construction of q-analogs; for instance, in the theory of basic hypergeometric series, it plays the role that the ordinary Pochhammer symbol plays in the theory of generalized hypergeometric series.
Unlike the ordinary Pochhammer symbol, the q-Pochhammer symbol can be extended to an infinite product:
The finite product can be expressed in terms of the infinite product:
which extends the definition to negative integers n. Thus, for nonnegative n, one has
The q-Pochhammer symbol is the subject of a number of q-series identities, particularly the infinite series expansions
which are both special cases of the q-binomial theorem:
The q-Pochhammer symbol is closely related to the enumerative combinatorics of partitions. The coefficient of in
is the number of partitions of m into at most n parts.
Since, by conjugation of partitions, this is the same as the number of partitions of m into parts of size at most n, by identification of generating series we obtain the identity:
as in the above section.
We also have that the coefficient of in
is the number of partitions of m into n or n-1 distinct parts.
By removing a triangular partition with n − 1 parts from such a partition, we are left with an arbitrary partition with at most n parts. This gives a weight-preserving bijection between the set of partitions into n or n − 1 distinct parts and the set of pairs consisting of a triangular partition having n − 1 parts and a partition with at most n parts. By identifying generating series, this leads to the identity:
also described in the above section.
The q-binomial theorem itself can also be handled by a slightly more involved combinatorial argument of a similar flavour.
Multiple arguments convention
Since identities involving q-Pochhammer symbols so frequently involve products of many symbols, the standard convention is to write a product as a single symbol of multiple arguments:
A q-series is a series in which the coefficients are functions of q, typically expressions of . Early results are due to Euler, Gauss, and Cauchy. The systematic study begins with E. Heinle (1943). 
Relationship to other q-functions
we define the q-analog of n, also known as the q-bracket or q-number of n to be
From this one can define the q-analog of the factorial, the q-factorial, as
Again, one recovers the usual factorial by taking the limit as q approaches 1. This can be interpreted as the number of flags in an n-dimensional vector space over the field with q elements, and taking the limit as q goes to 1 yields the interpretation of an ordering on a set as a flag in a vector space over the field with one element.
A product of negative integer q-brackets can be expressed in terms of the q-factorial as:
From the q-factorials, one can move on to define the q-binomial coefficients, also known as Gaussian coefficients, Gaussian polynomials, or Gaussian binomial coefficients:
One can check that
This converges to the usual Gamma function as q approaches 1 from inside the unit disc. Note that
for any x and
for non-negative integer values of n. Alternatively, this may be taken as an extension of the q-factorial function to the real number system.
- Basic hypergeometric series
- Elliptic gamma function
- Jacobi theta function
- Pochhammer symbol
- q-theta function
- George Gasper and Mizan Rahman, Basic Hypergeometric Series, 2nd Edition, (2004), Encyclopedia of Mathematics and Its Applications, 96, Cambridge University Press, Cambridge. ISBN 0-521-83357-4.
- Roelof Koekoek and Rene F. Swarttouw, The Askey scheme of orthogonal polynomials and its q-analogues, section 0.2.
- Exton, H. (1983), q-Hypergeometric Functions and Applications, New York: Halstead Press, Chichester: Ellis Horwood, 1983, ISBN 0853124914, ISBN 0470274530, ISBN 978-0470274538
- Weisstein, Eric W., "q-Analog", MathWorld.
- Weisstein, Eric W., "q-Bracket", MathWorld.
- Weisstein, Eric W., "q-Factorial", MathWorld.
- Weisstein, Eric W., "q-Series", MathWorld.
- Weisstein, Eric W., "q-Binomial Coefficient", MathWorld.
- Bruce C. Berndt, What is a q-series?, in Ramanujan Rediscovered: Proceedings of a Conference on Elliptic Functions, Partitions, and q-Series in memory of K. Venkatachaliengar: Bangalore, 1-5 June, 2009, N. D. Baruah, B. C. Berndt, S. Cooper, T. Huber, and M. J. Schlosser, eds., Ramanujan Mathematical Society, Mysore, 2010, pp. 31-51
- E. Heinle, Untersuchungen über die Reihe, J. Reine Angew. Math. 34 (1847), 285-328