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Hilbert’s nullstellensatz

26 November, 2007 in expository, math.AC, math.AG | Tags: , , | by | 35 comments

I had occasion recently to look up the proof of Hilbert’s nullstellensatz, which I haven’t studied since cramming for my algebra qualifying exam as a graduate student. I was a little unsatisfied with the proofs I was able to locate – they were fairly abstract and used a certain amount of algebraic machinery, which I was terribly rusty on – so, as an exercise, I tried to find a more computational proof that avoided as much abstract machinery as possible. I found a proof which used only the extended Euclidean algorithm and high school algebra, together with an induction on dimension and the obvious observation that any non-zero polynomial of one variable on an algebraically closed field has at least one non-root. It probably isn’t new (in particular, it might be related to the standard model-theoretic proof of the nullstellensatz, with the Euclidean algorithm and high school algebra taking the place of quantifier elimination), but I thought I’d share it here anyway.

Throughout this post, F is going to be a fixed algebraically closed field (e.g. the complex numbers {\Bbb C}). I’d like to phrase the nullstellensatz in a fairly concrete fashion, in terms of the problem of solving a set of simultaneous polynomial equations P_1(x) = \ldots = P_m(x) = 0 in several variables x = (x_1,\ldots,x_d) \in F^d over F, thus P_1,\ldots,P_m \in F[x] are polynomials in d variables. One obvious obstruction to solvability of this system is if the equations one is trying to solve are inconsistent in the sense that they can be used to imply 1=0. In particular, if one can find polynomials Q_1,\ldots,Q_m \in F[x] such that P_1 Q_1 + \ldots + P_m Q_m = 1, then clearly one cannot solve P_1(x)=\ldots=P_m(x)=0. The weak nullstellensatz asserts that this is, in fact, the only obstruction:

Weak nullstellensatz. Let P_1,\ldots,P_m \in F[x] be polynomials. Then exactly one of the following statements holds:

  1. The system of equations P_1(x)=\ldots=P_m(x)=0 has a solution x \in F^d.
  2. There exist polynomials Q_1,\ldots,Q_m \in F[x] such that P_1 Q_1 + \ldots + P_m Q_m = 1.

Note that the hypothesis that F is algebraically closed is crucial; for instance, if F is the reals, then the equation x^2+1=0 has no solution, but there is no polynomial Q(x) such that (x^2+1) Q(x) = 1.

Like many results of the “The only obstructions are the obvious obstructions” type, the power of the nullstellensatz lies in the ability to take a hypothesis about non-existence (in this case, non-existence of solutions to P_1(x)=\ldots=P_m(x)=0) and deduce a conclusion about existence (in this case, existence of Q_1,\ldots,Q_m such that P_1 Q_1 + \ldots + P_m Q_m = 1). The ability to get “something from nothing” is clearly going to be both non-trivial and useful. In particular, the nullstellensatz offers an important correspondence between algebraic geometry (the conclusion 1 is an assertion that a certain algebraic variety is empty) and commutative algebra (the conclusion 2 is an assertion that a certain ideal is non-proper).

Now suppose one is trying to solve the more complicated system P_1(x)=\ldots=P_d(x)=0; R(x) \neq 0 for some polynomials P_1,\ldots,P_d, R. Again, any identity of the form P_1 Q_1 + \ldots + P_m Q_m = 1 will be an obstruction to solvability, but now more obstructions are possible: any identity of the form P_1 Q_1 + \ldots + P_m Q_m = R^r for some non-negative integer r will also obstruct solvability. The strong nullstellensatz asserts that this is the only obstruction:

Strong nullstellensatz. Let P_1,\ldots,P_m, R \in F[x] be polynomials. Then exactly one of the following statements holds:

  1. The system of equations P_1(x)=\ldots=P_m(x)=0, R(x) \neq 0 has a solution x \in F^d.
  2. There exist polynomials Q_1,\ldots,Q_m \in F[x] and a non-negative integer r such that P_1 Q_1 + \ldots + P_m Q_m = R^r.

Of course, the weak nullstellensatz corresponds to the special case in which R=1. The strong nullstellensatz is usually phrased instead in terms of ideals and radicals, but the above formulation is easily shown to be equivalent to the usual version (modulo Hilbert’s basis theorem).

One could consider generalising the nullstellensatz a little further by considering systems of the form P_1(x)=\ldots=P_m(x)=0, R_1(x),\ldots,R_n(x) \neq 0, but this is not a significant generalisation, since all the inequations R_1(x) \neq 0, \ldots, R_n(x) \neq 0 can be concatenated into a single inequation R_1(x) \ldots R_n(x) \neq 0. The presence of the exponent r in conclusion (2) is a little annoying; to get rid of it, one needs to generalise the notion of an algebraic variety to that of a scheme (which is worth doing for several other reasons too, in particular one can now work over much more general objects than just algebraically closed fields), but that is a whole story in itself (and one that I am not really qualified to tell).

[Update, Nov 26: It turns out that my approach is more complicated than I first thought, and so I had to revise the proof quite a bit to fix a certain gap, in particular making it significantly messier than my first version. On the plus side, I was able to at least eliminate any appeal to Hilbert’s basis theorem, so in particular the proof is now manifestly effective (but with terrible bounds). In any case, I am keeping the argument here in case it has some interest.]

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