Eisenstein's criterion


In mathematics, Eisenstein's criterion gives a sufficient condition for a polynomial with integer coefficients to be irreducible over the rational numbers — that is, for it to not be factorizable into the product of non-constant polynomials with rational coefficients.
This criterion is not applicable to all polynomials with integer coefficients that are irreducible over the rational numbers, but it does allow in certain important cases for irreducibility to be proved with very little effort. It may apply either directly or after transformation of the original polynomial.
This criterion is named after Gotthold Eisenstein. In the early 20th century, it was also known as the Schönemann–Eisenstein theorem because Theodor Schönemann was the first to publish it.

Criterion

Suppose we have the following polynomial with integer coefficients.
If there exists a prime number such that the following three conditions all apply:
then is irreducible over the rational numbers. It will also be irreducible over the integers, unless all its coefficients have a nontrivial factor in common. The latter possibility can be avoided by first making primitive, by dividing it by the greatest common divisor of its coefficients. This division does not change whether is reducible or not over the rational numbers, and will not invalidate the hypotheses of the criterion for .

Examples

Eisenstein's criterion may apply either directly or after transformation of the original polynomial.

Direct (without transformation)

Consider the polynomial. In order for Eisenstein's criterion to apply for a prime number it must divide both non-leading coefficients and, which means only could work, and indeed it does since does not divide the leading coefficient, and its square does not divide the constant coefficient. One may therefore conclude that is irreducible over . Note that since is of degree 4, this conclusion could not have been established by only checking that has no rational roots, since a decomposition into two quadratic factors could also be possible.

Indirect (after transformation)

Often Eisenstein's criterion does not apply for any prime number. It may however be that it applies to the polynomial obtained after substitution of for. The fact that the polynomial after substitution is irreducible then allows concluding that the original polynomial is as well. This procedure is known as applying a shift.
For example consider, in which the coefficient 1 of is not divisible by any prime, Eisenstein's criterion does not apply to. But if one substitutes for in, one obtains the polynomial, which satisfies Eisenstein's criterion for the prime number. Since the substitution is an automorphism of the ring, the fact that we obtain an irreducible polynomial after substitution implies that we had an irreducible polynomial originally. In this particular example it would have been simpler to argue that could only be reducible if it had an integer root, which it obviously does not; however the general principle of trying substitutions in order to make Eisenstein's criterion apply is a useful way to broaden its scope.
Another possibility to transform a polynomial so as to satisfy the criterion, which may be combined with applying a shift, is reversing the order of its coefficients, provided its constant term is nonzero. This is so because such polynomials are reducible in if and only if they are reducible in , and in that ring the substitution of for reverses the order of the coefficients. As an example satisfies the criterion for after reversing its coefficients, and is therefore irreducible in.

Cyclotomic polynomials

An important class of polynomials whose irreducibility can be established using Eisenstein's criterion is that of the cyclotomic polynomials for prime numbers. Such a polynomial is obtained by dividing the polynomial by the linear factor, corresponding to its obvious root :
Here, as in the earlier example of, the coefficients prevent Eisenstein's criterion from applying directly. However the polynomial will satisfy the criterion for after substitution of for : this gives
all of whose non-leading coefficients are divisible by by properties of binomial coefficients, and whose constant coefficient is equal to, and therefore not divisible by. An alternative way to arrive at this conclusion is to use the identity which is valid in characteristic , to compute the reduction modulo of the quotient of polynomials:
which means that the non-leading coefficients of the quotient are all divisible by ; the remaining verification that the constant term of the quotient is can be done by substituting for into the expanded form.

History

Theodor Schönemann was the first to publish a version of the criterion, in 1846 in Crelle's Journal, which reads in translation
That will be irreducible to the modulus when to the modulus does not contain a factor.

This formulation already incorporates a shift to in place of ; the condition on means that is not divisible by, and so is divisible by but not by. As stated it is not entirely correct in that it makes no assumptions on the degree of the polynomial, so that the polynomial considered need not be of the degree that its expression suggests; the example, shows the conclusion is not valid without such hypothesis. Assuming that the degree of does not exceed, the criterion is correct however, and somewhat stronger than the formulation given above, since if is irreducible modulo , it certainly cannot decompose in into non-constant factors.
Subsequently Eisenstein published a somewhat different version in 1850, also in Crelle's Journal. This version reads in translation
When in a polynomial in of arbitrary degree the coefficient of the highest term is, and all following coefficients are whole numbers, into which a certain prime number divides, and when furthermore the last coefficient is equal to, where denotes a number not divisible by : then it is impossible to bring into the form
where,, and all and are whole numbers; the equation is therefore irreducible.

Here "whole real numbers" are ordinary integers and "whole complex numbers" are Gaussian integers; one should similarly interpret "real and complex prime numbers". The application for which Eisenstein developed his criterion was establishing the irreducibility of certain polynomials with coefficients in the Gaussian integers that arise in the study of the division of the lemniscate into pieces of equal arc-length.
Remarkably Schönemann and Eisenstein, once having formulated their respective criteria for irreducibility, both immediately apply it to give an elementary proof of the irreducibility of the cyclotomic polynomials for prime numbers, a result that Gauss had obtained in his Disquisitiones Arithmeticae with a much more complicated proof. In fact, Eisenstein adds in a footnote that the only proof for this irreducibility known to him, other than that of Gauss, is one given by Kronecker in 1845. This shows that he was unaware of the two different proofs of this statement that Schönemann had given in his 1846 article, where the second proof was based on the above-mentioned criterion. This is all the more surprising given the fact that two pages further, Eisenstein actually refers to the first part of Schönemann's article. In a note that appeared in the following issue of the Journal, Schönemann points this out to Eisenstein, and indicates that the latter's method is not essentially different from the one he used in the second proof.

Basic proof

To prove the validity of the criterion, suppose satisfies the criterion for the prime number, but that it is nevertheless reducible in, from which we wish to obtain a contradiction. From Gauss' lemma it follows that is reducible in as well, and in fact can be written as the product of two non-constant polynomials . Now reduce modulo to obtain a decomposition in. But by hypothesis this reduction for leaves its leading term, of the form for a non-zero constant, as the only nonzero term. But then necessarily the reductions modulo of and also make all non-leading terms vanish, since no other decompositions of are possible in, which is a unique factorization domain. In particular the constant terms of and vanish in the reduction, so they are divisible by, but then the constant term of, which is their product, is divisible by, contrary to the hypothesis, and one has a contradiction.
A second proof of Eisenstein's criterion also starts with the assumption that the polynomial is reducible. It is shown that this assumption entails a contradiction.
The assumption that
is reducible means that there are polynomials
Such that
The coefficient of the polynomial can be divided by the prime but not by. Since, it is possible to divide or by, but not both. One may without loss of generality proceed
By the assumption, does not divide. Because, neither nor can be divided by. Thus, if is the -th coefficient of the reducible polynomial, then
wherein cannot be divided by, because neither nor can be divided by.
We will prove that are all divisible by. As is also divisible by , this implies that
is divisible by, a contradiction proving the criterion.
It is possible to divide by, because can be divided by.
By initial assumption, it is possible to divide the coefficient of the polynomial by. Since
and since is not a multiple of it must be possible to divide by. Analogously, by induction, is a multiple of for all, which finishes the proof.

Advanced explanation

Applying the theory of the Newton polygon for the -adic number field, for an Eisenstein polynomial, we are supposed to take the lower convex envelope of the points
where is the -adic valuation of . Now the data we are given on the for, namely that they are at least one, is just what we need to conclude that the lower convex envelope is exactly the single line segment from to, the slope being.
This tells us that each root of has -adic valuation and hence that is irreducible over the -adic field ; and a fortiori over the rational number field.
This argument is much more complicated than the direct argument by reduction mod. It does however allow one to see, in terms of algebraic number theory, how frequently Eisenstein's criterion might apply, after some change of variable; and so limit severely the possible choices of with respect to which the polynomial could have an Eisenstein translate.
In fact only primes ramifying in the extension of generated by a root of have any chance of working. These can be found in terms of the discriminant of. For example, in the case given above, the discriminant is so that is the only prime that has a chance of making it satisfy the criterion. Modulo, it becomes — a repeated root is inevitable, since the discriminant is. Therefore the variable shift is actually something predictable.
Again, for the cyclotomic polynomial, it becomes
the discriminant can be shown to be , by linear algebra methods.
More precisely, only totally ramified primes have a chance of being Eisenstein primes for the polynomial. In fact, Eisenstein polynomials are directly linked to totally ramified primes, as follows: if a field extension of the rationals is generated by the root of a polynomial that is Eisenstein at then is totally ramified in the extension, and conversely if is totally ramified in a number field then the field is generated by the root of an Eisenstein polynomial at.

Generalization

Generalized criterion

Given an integral domain, let
be an element of, the polynomial ring with coefficients in.
Suppose there exists a prime ideal of such that
Then cannot be written as a product of two non-constant polynomials in. If in addition is primitive, then it is irreducible in. If is a unique factorization domain with field of fractions, then by Gauss's lemma is irreducible in, whether or not it is primitive ; in this case a possible choice of prime ideal is the principal ideal generated by any irreducible element of. The latter statement gives original theorem for or for.

Proof

The proof of this generalization is similar to the one for the original statement, considering the reduction of the coefficients modulo ; the essential point is that a single-term polynomial over the integral domain cannot decompose as a product in which at least one of the factors has more than one term.

Example

After, one of the basic examples of an integral domain is the polynomial ring in the variable over the field. In this case, the principal ideal generated by is a prime ideal. Eisenstein's criterion can then be used to prove the irreducibility of a polynomial such as in. Indeed, does not divide, does not divide, and divides and. This shows that this polynomial satisfies the hypotheses of the generalization of Eisenstein's criterion for the prime ideal since, for a principal ideal, being an element of is equivalent to being divisible by.