PARTITION REGULARITY AND THE PRIMESTHÁI HOÀNG LÊ

Abstract. The purpose of this note is to point out, as a simple yet nice consequence ofGreen and Tao’s program on counting linear patterns in the primes and Deuber’s work on
partition regularity, that if a system of equations is partition regular over the positive integers,then it is also partition regular over the sets {p − 1 : p prime} as well as {p + 1 : p prime}.
This answers a question of Li and Pan.

1. Introduction
Earliest results in arithmetic Ramsey theory include Schur’s theorem and van der Waerden’s
theorem. In 1916 Schur proved that if the positive integers are finitely colored (or partitionned), then there are x, y, z in the same color class such that x + y = z. In 1927 van derWaerden proved that if the positive integers are finitely colored, then for any k, we can finda monochromatic arithmetic progression of length k. We can even require further that thestep of the arithmetic progression belong to the same color class (Brauer [2]). More generally,given a u × v matrix with integer entries A, we say that A is (kernel) partition regular over aset X ⊂ Z \ {0} if under any finite coloring of X, we can always find a vector ~x ∈ X v , all of
whose components are of the same color, such that A~x = ~0.
The problem of determining all partition regular matrices over Z+ was settled by Rado [13].
He proved that A is partition regular over Z+ if and only if it satisfies the columns condition.
Let the columns of A be c~1 , . . . , ~cv . Then A satisfies the columns condition if there existm ∈ {1, 2, ..., v} and a partition {I1 , I2 , ..., Im } of {1, 2, ..., v} into nonempty sets such thatP(1)ci = ~0.
i∈I1 ~P(2) for each t ∈ {2, . . . , m}, i∈It ~ci is a linear combination over Q of {~ci : i ∈ ∪t−1
j=1 Ij }.
On the other hand, the closely related question of determining linear forms that can be foundin color classes was addressed by Hindman and Leader [11]. They completely determinedv × u matrices B such that under any finite coloring of Z+ we can always find a vector ~y ∈ Zu
such that the components of B~y are monochromatic. Such matrices are called image partition
regular (over Z+ ). We refer to the survey of Hindman [10] in which problems on this theme
are discussed in depth.
Let us call a set X ⊂ Z large if it contains a solution to all partition regular matrix overZ+ (that is, for any partition regular matrix A over Z+ , there exists a vector ~x, all of whose
components are in X, such that A~x = ~0). Thus Rado’s theorem says that under any finite
partition of Z+ , one of the sets is large. Rado conjectured the following, which was confirmed
by Deuber:
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THÁI HOÀNG LÊ

Theorem 1. If a large set is partitionned into a finite number of sets, then again one of thesesets is large.
In his resolution of Rado’s conjecture, Deuber introduced the notion of (m, p, c)-sets. Given
positive integers m, p, c, x1 , . . . , xm , the (m, p, c)-set generated by x1 , . . . , xm is the set of allnumbers of the form cxi + λi+1 xi+1 + . . . λm xm where 1 ≤ i ≤ m, −p ≤ λj ≤ p for all
j = i + 1, . . . , m. Thus an (m, p, c)-set is a generalization of an arithmetic progression, as wellas the sumset of a finite set.
One of the ingredients of Deuber’s resolution of Rado’s conjecture is the following
Theorem 2. A set X ⊂ Z is large if and only if it contains an (m, p, c)-set for every m, p, c ∈Z+ .
We now connect these results to the primes. Let P be the set of all primes. The following is a
deep result of Green and Tao’s program on counting linear patterns in the primes (Green-Tao[7, 8], Green-Tao-Ziegler [9]):Theorem
3. Let ψ1 , . . . , ψl : Zr → Zl be affine-linear forms with integer coefficients, ψi (~x) =Prn ∈ Zr such thatj=1 mi,j xj + ci , no two of which are affinely dependent. Then there exists ~ψ1 (~n), . . . , ψl (~n) are all primes if and only if for any k ∈ Z, k ≥ 2, there exists an ~x ∈ Zrsuch that ψ1 (~x), . . . , ψl (~x) are all non-divisible by k.
From this we easily deduce
Corollary 1. The sets P − 1 = {p − 1 : p prime} and P + 1 = {p + 1 : p prime} contain an(m, p, c)-set for every m, p, c ∈ Z+ .
By Theorem 2, this implies that the sets P − 1 and P + 1 are large. By Theorem 1, this meansthat under any finite coloring of P − 1 (or P + 1), there exists a color class that is large. Thus
we have proved the following:
Theorem 4. If a matrix A is partition regular over Z+ , then it is partition regular over P − 1and P + 1.
As a consequence, we have
Corollary 2. If the primes are finitely colored, then for any k we can find a monochromaticarithmetic progression of length k. Furthermore, if d is the step of the arithmetic progression,then we can require that d + 1 be prime, and of the same color as the arithmetic progression.
Corollary 2 is Conjecture 1.1 in Li-Pan [12]. In the same paper, Li and Pan proved thecase k = 2, based on Green’s transference principle [5], coupled with a quantitative version of
Schur’s theorem. In the same spirit, one can use a coloring version of Green-Tao’s transference
principle in [6], which is readily available [3, Lemma 2.6], coupled with a quantitative versionof Theorem 1, to prove a quantitative version of Theorem 4. It is more convenient to statethis quantitative version in terms of image partition regular matrices.

PARTITION REGULARITY AND THE PRIMES

3Theorem 5.Let B : Zr → Zl be an image partition regular matrix. For a vector ~x ∈ Zr ,
ψ1 (~x) ψ2 (~x) write B~x = 
. Then, under any finite coloring of P − 1, there is a color class C..
.
ψl (~x)such that
Xf (ψ1 (~x)) · · · f (ψl (~x))  N r+l~x∈Zr ,k~xk∞ ≤Nwhere f (x) = 1C (x)Λ(x + 1) and Λ is the von Mangoldt function. Here the implied constant
only depends on B and the number of colors used.
In other words, the number of monochromatic configurations ψ1 (~x), · · · , ψl (~x) is as large as
one can hope for in order of magnitude. We won’t prove Theorem 5 since it’s only slightly
stronger than Theorem 4, and utilizes the same machinery (specifically, the fact that (a variantof) the function Λ is very close to 1 in Gowers norms).
Acknowledgements
The author would like to thank Terence Tao for helpful discussions and encouragement.
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Department of Mathematics The University of Texas at Austin, 1 University Station, C1200
Austin, Texas 78712.
E-mail address: leth@math.utexas.edu