q-Series

A q-series is series involving coefficients of the form

(a;q)_n=product_(k=0)^(n-1)(1-aq^k)
(1)
=product_(k=0)^(infty)((1-aq^k))/((1-aq^(k+n)))
(2)
=((a;q)_infty)/((aq^n;q)_infty)
(3)

for n>=1, where (a;q)_infty is defined as

 (a;q)_infty=product_(k=0)^infty(1-aq^k).
(4)

The symbol (a;q)_infty is called a q-Pochhammer symbol (Andrews 1986, p. 10) since it is a q-analog of the usual Pochhammer symbol. q-series obey beautifully sets of properties, and arise naturally in the theory of partitions, as well as in many problems of mathematical physics, especially those enumerating possible numbers of configurations or states on a lattice. The shorthand notation

 (a)_n=(a;q)_n
(5)

is commonly encountered, and the notation

 (q)_n=(q;q)_n=product_(k=1)^n(1-q^k)
(6)

is another special case (Hirschhorn 1999).

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