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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 nonzero 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 padic numbers: with an appropriate extension of the idea, the padic 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 nonzero. Retrieved from "http://en.wikipedia.org/"

