1 Hilbert ’ s Tenth Problem

Hilbert’s 10th problem, stated in modern terms, is Find an algorithm that will, given p ∈ Z[x1, . , xn], determine if there exists a1, . , an ∈ Z such that p(a1, . . . , an) = 0. Davis, Putnam, Robinson, and Matiyasevich showed that there is no such algorithm. But what if we bound the degree of the polynomial? The number of variables? This paper surveys what is known for these cases. 1 Hilbert’s Tenth Problem In 1900 Hilbert proposed 23 problems for mathematicians to work on over the next 100 years (or longer). The 10th problem, stated in modern terms, is Find an algorithm that will, given p ∈ Z[x1, . , xn], determine if there exists a1, . , an ∈ Z such that p(a1, . . . , an) = 0. Hilbert probably thought this would inspire much deep number theory, and it did inspire some. But the work on this problem took a very different direction. Davis, Putnam, and Robinson [9] showed that determining if an exponential diophantine equation has a solution in Z is undecidable. Their proof coded Turing machines into such equations. Matiyasevich [20] extended their work by showing how to replace the exponentials with polynomials. Hence the algorithm that Hilbert wanted is not possible. For a self contained proof from soup to nuts see Davis’ exposition [8]. For more about both the proof and the implications of the result see the book of Matiyasevich [21]. This raises the obvious question of what happens for particular numbers of variables n and degree d. I thought that surely there must be a grid on the web where the d-n-th entry is • D if the problem for degree ≤ d, and ≤ n variables is Decidable. ∗The University of Maryland at College Park, gasarch@umd.edu • U if the problem for degree ≤ d, and ≤ n variables is Undecidable. • ? if the status of the problem for degree ≤ d, and ≤ n variables is unknown. There is a graph in a German paper, by Bayer et al. [4], that has the information I want, though it’s hard to read. So putting that aside I ask Why has the quest for a grid not gotten more attention? Here are some speculations. 1. Logicians work on showing that determining if there is a solution in N is undecidable. Number theorists worked on showing that determining if there is a solution in Z is decidable. Since Logicians worked in N and Number Theorists in Z, a grid would need to reconcile these two related problems. 2. There is a real dearth of positive results, so a grid would not be that interesting. 3. The undecidable results often involve rather large values of d, so the grid would be hard to draw. 4. Timothy Chow offered this speculation in an email to me: One reason there isn’t already a website of the type you envision is that from a numbertheoretic (or decidability) point of view, parameterization by degree and number of variables is not as natural as it might seem at first glance. The most fruitful lines of research have been geometric, and so geometric concepts such as smoothness, dimension, and genus are more natural than, say, degree. A nice survey by a number theorist is the book Rational Points on Varieties by Bjorn Poonen [25]. Much of it is highly technical; however, reading the preface is very enlightening. Roughly speaking, the current state of the art is that there is really only one known way to prove that a system of Diophantine equations has no rational solution. An alternative to a grid is a paper to collect up all that is known and point to open problems. This article is that paper. None of the results are mine. Another alternative is to have a table that gives, for each n, the largest value of d for which the problem is decidable, and the smallest value of d for which the problem is known to be undecidable. Anyone can take this paper and create such a table. In Section 2 we will relate the problem of seeking solutions in Z with the problem of seeking solutions in N. In Section 3 we will present values of (d, n) where the problem is undecidable. In Section 4 we will present values of (d, n) where the problem is decidable. In Sections 5 and 6 we look at restricted sets of polynomials. In Section 7 we will discuss the vast area between the decidable and undecidable cases. In Section 8 we will briefly present Matiyasevich’s discussion of what Hilbert really wanted in contrast to what happened. 2 Definitions and Reconciling N with Z Notation 2.1. 1. HZ(d, n) is the problem where the degree is ≤ d, the number of variables is ≤ n, and we seek a solution in Z. 2. HN(d, n) is the problem where the degree is ≤ d, the number of variables is ≤ n, and we seek a solution in N. 3. HZ(d, n) = D means that there is an algorithm to decide HZ(d, n). 4. HZ(d, n) = U means that there is no algorithm to decide HZ(d, n). 5. Similarly for HN(d, n) equal to D or U. The four parts of the next lemma are usually stated with x1, x2, x3 ∈ N or x1, x2, x3, x4 ∈ N, and not in the iff form we use. However, we need these statements in the form we present them. Lemma 2.2. 1. x ∈ N and x is not of the form 4a(8b + 7) (where a, b ∈ N) iff there exists x1, x2, x3 ∈ Z such that x = x 1 + x 2 2 + x 2 3. 2. x ∈ N and x ≡ 1 (mod 4) iff there exists x1, x2, x3 ∈ Z such that (1) x1, x2 ≡ 0 (mod 2), (2) x3 ≡ 1 (mod 2), and x = x 1 + x 2 2 + x 2 3. 3. n ∈ N iff there exists x1, x2, x3 ∈ Z such that n = x2 1 + x2 2 + x2 3 + x3. Proof. 1) This is Legendre’s three-square theorem. It is sometimes called the GaussLegendre Theorem. 2) Since x ≡ 1 (mod 4), x satisfies the hypothesis of Part 1. Hence there exists x1, x2, x3 such that x = x 1 + x 2 2 + x 2 3. Take this equation mod 4. 1 ≡ x 1 + x 2 + x 3 (mod 4). It is easy to see that the only parities of x1, x2, x3 that work are for two of them to be even and one of them to be odd. 3) Let n ∈ N. Note that 4n+1 satisfies the premise of Part 2. By Part 2 there exists x1, x2, x3 ∈ Z such that 4n + 1 = (2x1) + (2x2) + (2x3 + 1) 4n + 1 = 4x 1 + 4x 2 2 + 4x 2 3 + 4x3 + 1 n = x 1 + x 2 2 + x 2 3 + x3.