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The -binomial coefficient is a q-analog for the binomial coefficient, also called a Gaussian coefficient or a Gaussian polynomial. A -binomial coefficient is given by
(1) |
where
(2) |
is a q-series (Koepf 1998, p. 26). For ,
(3) |
where is a q-factorial (Koepf 1998, p. 30). The -binomial coefficient can also be defined in terms of the q-brackets by
(4) |
The -binomial is implemented in the Wolfram Language as QBinomial[n, m, q].
For , the -binomial coefficients turn into the usual binomial coefficient.
The special case
(5) |
is sometimes known as the q-bracket.
The -binomial coefficient satisfies the recurrence equation
(6) |
for all and , so every -binomial coefficient is a polynomial in . The first few -binomial coefficients are
(7) |
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(8) |
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(9) |
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(10) |
From the definition, it follows that
(11) |
Additional identities include
(12) |
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(13) |
The -binomial coefficient can be constructed by building all -subsets of , summing the elements of each subset, and taking the sum
(14) |
over all subset-sums (Kac and Cheung 2001, p. 19).
The -binomial coefficient can also be interpreted as a polynomial in whose coefficient counts the number of distinct partitions of elements which fit inside an rectangle. For example, the partitions of 1, 2, 3, and 4 are given in the following table.
partitions | |
0 | |
1 | |
2 | |
3 | |
4 |
Of these, , , , , , and fit inside a box. The counts of these having 0, 1, 2, 3, and 4 elements are 1, 1, 2, 1, and 1, so the (4, 2)-binomial coefficient is given by
(15) |
as above.
REFERENCES:
Gasper, G. and Rahman, M. Basic Hypergeometric Series. Cambridge, England: Cambridge University Press, 1990.
Kac, V. Cheung, P. Quantum Calculus. New York:Springer-Verlag, 2001.
Koekoek, R. and Swarttouw, R. F. "The q-Gamma Function and the q-Binomial Coefficient." §0.3 in The Askey-Scheme of Hypergeometric Orthogonal Polynomials and its q-Analogue. Delft, Netherlands: Technische Universiteit Delft, Faculty of Technical Mathematics and Informatics Report 98-17, pp. 10-11, 1998.
Koepf, W. Hypergeometric Summation: An Algorithmic Approach to Summation and Special Function Identities. Braunschweig, Germany: Vieweg, p. 26, 1998.a
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