Eisenstein's theorem

In mathematics, Eisenstein's theorem, named after the German mathematician Ferdinand Eisenstein, applies to the coefficients of any power series which is an algebraic function with rational number coefficients, and it is a very powerful test. Through the theorem, it is readily demonstrable that a function such as the exponential function must be a transcendental function.

Suppose therefore that

is a formal power series with rational coefficients an, which has a non-zero radius of convergence in the complex plane, and within it represents an analytic function that is in fact an algebraic function. Let dn denote the denominator of an, as a fraction in lowest terms. Then Eisenstein's theorem states that there is a finite set S of prime numbers p, such that every prime factor of a number dn is contained in S.

This has an interpretation in terms of p-adic numbers: with an appropriate extension of the idea, the p-adic radius of convergence of the series is at least 1, for almost all p (i.e. the primes outside the finite set S). In fact that statement is a little weaker, in that it disregards any initial partial sum of the series, in a way that may vary according to p. For the other primes the radius is non-zero.