# Integer-valued polynomial

In mathematics, an integer-valued polynomial (also known as a numerical polynomial) P(t) is a polynomial whose value P(n) is an integer for every integer n. Every polynomial with integer coefficients is integer-valued, but the converse is not true. For example, the polynomial

${\displaystyle {\frac {1}{2}}t^{2}+{\frac {1}{2}}t={\frac {1}{2}}t(t+1)}$

takes on integer values whenever t is an integer. That is because one of t and t + 1 must be an even number. (The values this polynomial takes are the triangular numbers.)

Integer-valued polynomials are objects of study in their own right in algebra, and frequently appear in algebraic topology.[1]

## Classification

The class of integer-valued polynomials was described fully by Pólya (1915). Inside the polynomial ring Q[t] of polynomials with rational number coefficients, the subring of integer-valued polynomials is a free abelian group. It has as basis the polynomials

Pk(t) = t(t 1)...(t k + 1)/k!

for k = 0,1,2, ..., i.e., the binomial coefficients. In other words, every integer-valued polynomial can be written as an integer linear combination of binomial coefficients in exactly one way. The proof is by the method of discrete Taylor series: binomial coefficients are integer-valued polynomials, and conversely, the discrete difference of an integer series is an integer series, so the discrete Taylor series of an integer series generated by a polynomial has integer coefficients (and is a finite series).

## Fixed prime divisors

Integer-valued polynomials may be used effectively to solve questions about fixed divisors of polynomials. For example, the polynomials P with integer coefficients that always take on even number values are just those such that P/2 is integer valued. Those in turn are the polynomials that may be expressed as a linear combination with even integer coefficients of the binomial coefficients.

In questions of prime number theory, such as Schinzel's hypothesis H and the Bateman–Horn conjecture, it is a matter of basic importance to understand the case when P has no fixed prime divisor (this has been called Bunyakovsky's property, after Viktor Bunyakovsky). By writing P in terms of the binomial coefficients, we see the highest fixed prime divisor is also the highest prime common factor of the coefficients in such a representation. So Bunyakovsky's property is equivalent to coprime coefficients.

As an example, the pair of polynomials n and n2 + 2 violates this condition at p = 3: for every n the product

n(n2 + 2)

is divisible by 3, which follows from the representation

${\displaystyle n(n^{2}+2)=6{\binom {n}{3}}+6{\binom {n}{2}}+3{\binom {n}{1}}}$

with respect to the binomial basis, where the highest common factor of the coefficientshence the highest fixed divisor of n(n2 + 2)is 3.

## Other rings

Numerical polynomials can be defined over other rings and fields, in which case the integer-valued polynomials above are referred to as classical numerical polynomials.

## Applications

The K-theory of BU(n) is numerical (symmetric) polynomials.

The Hilbert polynomial of a polynomial ring in k + 1 variables is the numerical polynomial ${\displaystyle {\binom {t+k}{k}}}$.

## References

1. Johnson, Keith (2014), "Stable homotopy theory, formal group laws, and integer-valued polynomials", in Fontana, Marco; Frisch, Sophie; Glaz, Sarah (eds.), Commutative Algebra: Recent Advances in Commutative Rings, Integer-Valued Polynomials, and Polynomial Functions, Springer, pp. 213–224, ISBN 9781493909254. See in particular pp. 213–214.

### Algebra

• Cahen, P-J.; Chabert, J-L. (1997), Integer-valued polynomials, Mathematical Surveys and Monographs, 48, Providence, RI: American Mathematical Society
• Pólya, G. (1915), "Über ganzwertige ganze Funktionen", Palermo Rend. (in German), 40: 1–16, ISSN 0009-725X, JFM 45.0655.02

### Algebraic topology

• A. Baker; F. Clarke; N. Ray; L. Schwartz (1989), "On the Kummer congruences and the stable homotopy of BU", Trans. Amer. Math. Soc., Transactions of the American Mathematical Society, Vol. 316, No. 2, 316 (2): 385–432, doi:10.2307/2001355, JSTOR 2001355