Hensel's lemma

In mathematics, Hensel's lemma, also known as Hensel's lifting lemma, named after Kurt Hensel, is a result in modular arithmetic, stating that if a univariate polynomial has a simple root modulo a prime number, then this root can be lifted to a unique root modulo any higher power of. More generally, if a polynomial factors modulo into two coprime polynomials, this factorization can be lifted to a factorization modulo any higher power of .
By passing to the "limit" when the power of tends to infinity, it follows that a root or a factorization modulo can be lifted to a root or a factorization over the -adic integers.
These results have been widely generalized, under the same name, to the case of polynomials over an arbitrary commutative ring, where is replaced by an ideal, and "coprime polynomials" means "polynomials that generate an ideal containing ".
Hensel's lemma is fundamental in -adic analysis, a branch of analytic number theory.
The proof of Hensel's lemma is constructive, and leads to an efficient algorithm for Hensel lifting, which is fundamental for factoring polynomials, and gives the most efficient known algorithm for exact linear algebra over the rational numbers.

Modular reduction and lifting

Hensel's original lemma concerns the relation between polynomial factorization over the integers and over the integers modulo a prime number and its powers. It can be straightforwardly extended to the case where the integers are replaced by any commutative ring, and is replaced by any maximal ideal.
Making this precise requires a generalization of the usual modular arithmetic, and so it is useful to define accurately the terminology that is commonly used in this context.
Let be a commutative ring, and an ideal of. Reduction modulo refers to the replacement of every element of by its image under the canonical map For example, if is a polynomial with coefficients in, its reduction modulo, denoted is the polynomial in obtained by replacing the coefficients of by their image in Two polynomials and in are congruent modulo, denoted if they have the same coefficients modulo, that is if If a factorization of modulo consists in two polynomials in such that
The lifting process is the inverse of reduction. That is, given objects depending on elements of the lifting process replaces these elements by elements of that maps to them in a way that keeps the properties of the objects.
For example, given a polynomial and a factorization modulo expressed as lifting this factorization modulo consists of finding polynomials such that and Hensel's lemma asserts that such a lifting is always possible under mild conditions; see next section.

Statement

Originally, Hensel's lemma was stated for lifting a factorization modulo a prime number of a polynomial over the integers to a factorization modulo any power of and to a factorization over the -adic integers. This can be generalized easily, with the same proof to the case where the integers are replaced by any commutative ring, the prime number is replaced by a maximal ideal, and the -adic integers are replaced by the completion with respect to the maximal ideal. It is this generalization, which is also widely used, that is presented here.
Let be a maximal ideal of a commutative ring, and
be a polynomial in with a leading coefficient not in
Since is a maximal ideal, the quotient ring is a field, and is a principal ideal domain, and, in particular, a unique factorization domain, which means that every nonzero polynomial in can be factorized in a unique way as the product of a nonzero element of and irreducible polynomials that are monic.
Hensel's lemma asserts that every factorization of modulo into coprime polynomials can be lifted in a unique way into a factorization modulo for every.
More precisely, with the above hypotheses, if where and are monic and coprime modulo then, for every positive integer there are monic polynomials and such that
and and are unique modulo

Lifting simple roots

An important special case is when In this case the coprimality hypothesis means that is a simple root of This gives the following special case of Hensel's lemma, which is often also called Hensel's lemma.
With above hypotheses and notations, if is a simple root of then can be lifted in a unique way to a simple root of for every positive integer. Explicitly, for every positive integer, there is a unique such that and is a simple root of

Lifting to adic completion

The fact that one can lift to for every positive integer suggests to "pass to the limit" when tends to the infinity. This was one of the main motivations for introducing -adic integers.
Given a maximal ideal of a commutative ring, the powers of form a basis of open neighborhoods for a topology on, which is called the -adic topology. The completion of this topology can be identified with the completion of the local ring and with the inverse limit This completion is a complete local ring, generally denoted When is the ring of the integers, and where is a prime number, this completion is the ring of -adic integers
The definition of the completion as an inverse limit, and the above statement of Hensel's lemma imply that every factorization into pairwise coprime polynomials modulo of a polynomial can be uniquely lifted to a factorization of the image of in Similarly, every simple root of modulo can be lifted to a simple root of the image of in

Proof

Hensel's lemma is generally proved incrementally by lifting a factorization over to either a factorization over, or a factorization over .
The main ingredient of the proof is that coprime polynomials over a field satisfy Bézout's identity. That is, if and are coprime univariate polynomials over a field, there are polynomials and such that and
Bézout's identity allows defining coprime polynomials and proving Hensel's lemma, even if the ideal is not maximal. Therefore, in the following proofs, one starts from a commutative ring, an ideal, a polynomial that has a leading coefficient that is invertible modulo, and factorization of modulo or modulo a power of, such that the factors satisfy a Bézout's identity modulo. In these proofs, means

[|Linear lifting]

Let be an ideal of a commutative ring, and be a univariate polynomial with coefficients in that has a leading coefficient that is invertible modulo .
Suppose that for some positive integer there is a factorization
such that and are monic polynomials that are coprime modulo, in the sense that there exist such that Then, there are polynomials such that and
Under these conditions, and are unique modulo
Moreover, and satisfy the same Bézout's identity as and, that is, This follows immediately from the preceding assertions, but is needed to apply iteratively the result with increasing values of.
The proof that follows is written for computing and by using only polynomials with coefficients in or When and this allows manipulating only integers modulo.
Proof: By hypothesis, is invertible modulo. This means that there exists and such that
Let of degree less than such that
(One may choose but other choices may lead to simpler computations. For example, if and it is possible and better to choose where the coefficients of are integers in the interval
As is monic, the Euclidean division of by is defined, and provides and such that and Moreover, both and are in Similarly, let with and
One has Indeed, one has
As is monic, the degree modulo of can be less than only if
Thus, considering congruences modulo one has
So, the existence assertion is verified with

Uniqueness

Let,, and as a in the preceding section. Let
be a factorization into coprime polynomials, such The application of linear lifting for shows the existence of and such that and
The polynomials and are uniquely defined modulo This means that, if another pair satisfies the same conditions, then one has
Proof: Since a congruence modulo implies the same congruence modulo one can proceed by induction and suppose that the uniqueness has been proved for, the case being trivial. That is, one can suppose that
By hypothesis, has
and thus
By induction hypothesis, the second term of the latter sum belongs to and the same is thus true for the first term. As is invertible modulo, there exist and such that Thus
using the induction hypothesis again.
The coprimality modulo implies the existence of such that Using the induction hypothesis once more, one gets
Thus one has a polynomial of degree less than that is congruent modulo to the product of the monic polynomial and another polynomial. This is possible only if and implies Similarly, is also in and this proves the uniqueness.

[|Quadratic lifting]

Linear lifting allows lifting a factorization modulo to a factorization modulo Quadratic lifting allows lifting directly to a factorization modulo at the cost of lifting also the Bézout's identity and of computing modulo instead of modulo .
For lifting up to modulo for large one can use either method. If, say, a factorization modulo requires steps of linear lifting or only steps of quadratic lifting. However, in the latter case the size of the coefficients that have to be manipulated increase during the computation. This implies that the best lifting method depends on the context.
Quadratic lifting is based on the following property.
Suppose that for some positive integer there is a factorization
such that and are monic polynomials that are coprime modulo, in the sense that there exist such that Then, there are polynomials such that and
Moreover, and satisfy a Bézout's identity of the form
Proof: The first assertion is exactly that of linear lifting applied with to the ideal instead of
Let One has
where
Setting and one gets
which proves the second assertion.

Explicit example

Let
Modulo 2, Hensel's lemma cannot be applied since the reduction of modulo 2 is simply^{pg 15-16}
with 6 factors not being relatively prime to each other. By Eisenstein's criterion, however, one can conclude that the polynomial is irreducible in
Over, on the other hand, one has
where is the square root of 2 in. As 4 is not a cube in these two factors are irreducible over. Hence the complete factorization of in and is
where is a square root of 2 in that can be obtained by lifting the above factorization.
Finally, in the polynomial splits into
with all factors relatively prime to each other, so that in and there are 6 factors with the 727-adic integers

Using derivatives for lifting roots

Let be a polynomial with integer coefficients, and let m, k be positive integers such that m ≤ k. If r is an integer such that
then, for every there exists an integer s such that
Furthermore, this s is unique modulo p^k+''m, and can be computed explicitly as the integer such that
where is an integer satisfying
Note that so that the condition is met. As an aside, if, then 0, 1, or several s'' may exist.

Derivation

We use the Taylor expansion of f around r to write:
From we see that s − r = tp^k for some integer t. Let
For we have:
The assumption that is not divisible by p ensures that has an inverse mod which is necessarily unique. Hence a solution for t exists uniquely modulo and s exists uniquely modulo

Observations

Criterion for irreducible polynomials

Using the above hypotheses, if we consider an irreducible polynomial
such that, then
In particular, for, we find in
but, hence the polynomial cannot be irreducible. Whereas in we have both values agreeing, meaning the polynomial could be irreducible. In order to determine irreducibility, the Newton polygon must be employed.

Frobenius

Note that given an the Frobenius endomorphism gives a nonzero polynomial that has zero derivative
hence the pth roots of do not exist in. For, this implies that cannot contain the root of unity.

Roots of unity

Although the pth roots of unity are not contained in, there are solutions of. Note that
is never zero, so if there exists a solution, it necessarily lifts to. Because the Frobenius gives all of the non-zero elements are solutions. In fact, these are the only roots of unity contained in

Hensel lifting

Using the lemma, one can "lift" a root r of the polynomial f modulo p^k to a new root s modulo p^k+1 such that . In fact, a root modulo p^k+1 is also a root modulo p^k, so the roots modulo p^k+1 are precisely the liftings of roots modulo p^k. The new root s is congruent to r modulo p, so the new root also satisfies So the lifting can be repeated, and starting from a solution r_k of we can derive a sequence of solutions r_k+1, r_k+2,... of the same congruence for successively higher powers of p, provided that for the initial root r_k. This also shows that f has the same number of roots mod p^k as mod p^k+1, mod p^k+2, or any other higher power of p, provided that the roots of f mod p^k are all simple.
What happens to this process if r is not a simple root mod p? Suppose that
Then implies That is, for all integers t. Therefore, we have two cases:

If then there is no lifting of r to a root of f modulo p^k+1.
If then every lifting of r to modulus p^k+1 is a root of f modulo p^k+1.

Example. To see both cases we examine two different polynomials with :
and. Then and We have which means that no lifting of 1 to modulus 4 is a root of f modulo 4.
and. Then and However, since we can lift our solution to modulus 4 and both lifts are solutions. The derivative is still 0 modulo 2, so a priori we don't know whether we can lift them to modulo 8, but in fact we can, since g is 0 mod 8 and g is 0 mod 8, giving solutions at 1, 3, 5, and 7 mod 8. Since of these only g and g are 0 mod 16 we can lift only 1 and 7 to modulo 16, giving 1, 7, 9, and 15 mod 16. Of these, only 7 and 9 give, so these can be raised giving 7, 9, 23, and 25 mod 32. It turns out that for every integer, there are four liftings of 1 mod 2 to a root of.

Hensel's lemma for ''p''-adic numbers

In the -adic numbers, where we can make sense of rational numbers modulo powers of p as long as the denominator is not a multiple of p, the recursion from r_k to r_k+1 can be expressed in a much more intuitive way. Instead of choosing t to be an integer which solves the congruence
let t be the rational number :
Then set
This fraction may not be an integer, but it is a -adic integer, and the sequence of numbers r_k converges in the -adic integers to a root of f = 0. Moreover, the displayed recursive formula for the number r_k+1 in terms of r_k is precisely Newton's method for finding roots to equations in the real numbers.
By working directly in the -adics and using the -adic absolute value, there is a version of Hensel's lemma which can be applied even if we start with a solution of f ≡ 0 mod p such that We just need to make sure the number is not exactly 0. This more general version is as follows: if there is an integer a which satisfies:
then there is a unique -adic integer b such f = 0 and The construction of b amounts to showing that the recursion from Newton's method with initial value a converges in the -adics and we let b be the limit. The uniqueness of b as a root fitting the condition needs additional work.
The statement of Hensel's lemma given above is a special case of this more general version, since the conditions that f ≡ 0 mod p and say that and

Examples

Suppose that p is an odd prime and a is a non-zero quadratic residue modulo p. Then Hensel's lemma implies that a has a square root in the ring of -adic integers Indeed, let If r is a square root of a modulo p then:
where the second condition is dependent on the fact that p is odd. The basic version of Hensel's lemma tells us that starting from r₁ = r we can recursively construct a sequence of integers such that:
This sequence converges to some -adic integer b which satisfies b² = a. In fact, b is the unique square root of a in congruent to r₁ modulo p. Conversely, if a is a perfect square in and it is not divisible by p then it is a nonzero quadratic residue mod p. Note that the quadratic reciprocity law allows one to easily test whether a is a nonzero quadratic residue mod p, thus we get a practical way to determine which -adic numbers have a -adic square root, and it can be extended to cover the case p = 2 using the more general version of Hensel's lemma.
To make the discussion above more explicit, let us find a "square root of 2" in the 7-adic integers. Modulo 7 one solution is 3, so we set. Hensel's lemma then allows us to find as follows:
Based on which the expression
turns into:
which implies Now:
And sure enough,
We can continue and find. Each time we carry out the calculation, one more base 7 digit is added for the next higher power of 7. In the 7-adic integers this sequence converges, and the limit is a square root of 2 in which has initial 7-adic expansion
If we started with the initial choice then Hensel's lemma would produce a square root of 2 in which is congruent to 4 instead of 3 and in fact this second square root would be the negative of the first square root.
As an example where the original version of Hensel's lemma is not valid but the more general one is, let and Then and so
which implies there is a unique 2-adic integer b satisfying
i.e., b ≡ 1 mod 4. There are two square roots of 17 in the 2-adic integers, differing by a sign, and although they are congruent mod 2 they are not congruent mod 4. This is consistent with the general version of Hensel's lemma only giving us a unique 2-adic square root of 17 that is congruent to 1 mod 4 rather than mod 2. If we had started with the initial approximate root a = 3 then we could apply the more general Hensel's lemma again to find a unique 2-adic square root of 17 which is congruent to 3 mod 4. This is the other 2-adic square root of 17.
In terms of lifting the roots of from modulus 2^k to 2^k+1, the lifts starting with the root 1 mod 2 are as follows:
For every k at least 3, there are four roots of x² − 17 mod 2^k, but if we look at their 2-adic expansions we can see that in pairs they are converging to just two 2-adic limits. For instance, the four roots mod 32 break up into two pairs of roots which each look the same mod 16:
The 2-adic square roots of 17 have expansions
Another example where we can use the more general version of Hensel's lemma but not the basic version is a proof that any 3-adic integer c ≡ 1 mod 9 is a cube in Let and take initial approximation a = 1. The basic Hensel's lemma cannot be used to find roots of f since for every r. To apply the general version of Hensel's lemma we want which means That is, if c ≡ 1 mod 27 then the general Hensel's lemma tells us f has a 3-adic root, so c is a 3-adic cube. However, we wanted to have this result under the weaker condition that c ≡ 1 mod 9. If c ≡ 1 mod 9 then c ≡ 1, 10, or 19 mod 27. We can apply the general Hensel's lemma three times depending on the value of c mod 27: if c ≡ 1 mod 27 then use a = 1, if c ≡ 10 mod 27 then use a = 4, and if c ≡ 19 mod 27 then use a = 7.
In a similar way, after some preliminary work, Hensel's lemma can be used to show that for any odd prime number p, any -adic integer c congruent to 1 modulo p² is a p-th power in

Generalizations

Suppose A is a commutative ring, complete with respect to an ideal and let a ∈ A is called an "approximate root" of f, if
If f has an approximate root then it has an exact root b ∈ A "close to" a; that is,
Furthermore, if is not a zero-divisor then b is unique.
This result can be generalized to several variables as follows:
As a special case, if for all i and is a unit in A then there is a solution to f = 0 with for all i.
When n = 1, a = a is an element of A and The hypotheses of this multivariable Hensel's lemma reduce to the ones which were stated in the one-variable Hensel's lemma.

Related concepts

Completeness of a ring is not a necessary condition for the ring to have the Henselian property: Goro Azumaya in 1950 defined a commutative local ring satisfying the Henselian property for the maximal ideal m to be a Henselian ring.
Masayoshi Nagata proved in the 1950s that for any commutative local ring A with maximal ideal m there always exists a smallest ring A^h containing A such that A^h is Henselian with respect to mA^h. This A^h is called the Henselization of A. If A is noetherian, A^h will also be noetherian, and A^h is manifestly algebraic as it is constructed as a limit of étale neighbourhoods. This means that A^h is usually much smaller than the completion Â while still retaining the Henselian property and remaining in the same category.