# Denumerant

2010 Mathematics Subject Classification: Primary: 05A17 [MSN][ZBL]

The number $D(n; a_1,\ldots,a_m)$ of partitions of an integer $n$ into parts equal to $a_1,\ldots,a_m$, i.e. the number of solutions in non-negative integers $x_1,\ldots,x_m$ of the equation $$n = a_1 x_1 + \cdots + a_m x_m \ .$$ The generating function of the denumerants is $$D(z; a_1,\ldots,a_m) = \sum_n D(n;a_1,\ldots,a_m) z^n = \frac{1}{\left({1-z^{a_1}}\right)\cdots\left({1-z^{a_m}}\right)} \ .$$

The simplest method of computing a denumerant is by Euler's recurrence relation: $$D(n;1,\ldots,k) - D(n-k;1,\ldots,k) = D(n;1,\ldots,k-1) \ .$$

Explicit formulas for certain denumerants may be obtained from the following theorem: If $A$ is the least common multiple of the numbers $a_1,\ldots,a_m$, then the denumerant $$D(An+b;a_1,\ldots,a_m) \ ;\ \ b=0,\ldots,A-1$$ is a polynomial of degree $(m-1)$ with respect to $n$.

#### References

 [1] J. Riordan, "An introduction to combinational analysis" , Wiley [1958] Dover (2002) ISBN 0-486-42536-3 Zbl 0078.00805
How to Cite This Entry:
Denumerant. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Denumerant&oldid=41142
This article was adapted from an original article by V.E. Tarakanov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article