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Structure Theory of Set Addition

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Structure Theory of Set Addition Structure Theory of Set Addition Notes by B. J. Green1 ICMS Instructional Conference in Combinatorial Aspects of Mathematical Analysis, Edinburgh March 25 { April 5 2002. 1 Lecture 1: Pl¨unnecke's Inequalities 1.1 Introduction The object of these notes is to explain a recent proof by Ruzsa of a famous result of Freiman, some significant modifications of Ruzsa's proof due to Chang, and all the background material necessary to understand these arguments. Freiman's theorem concerns the structure of sets with small sumset. Let A be a subset of an abelian group G, and define the sumset A + A to be the set of all pairwise sums a + a0, where a; a0 are (not necessarily distinct) elements of A. If A = n then A + A n, and equality can occur (for example if A is a subgroup of G).j Inj the otherj directionj ≥ we have A + A n(n + 1)=2, and equality can occur here too, for example when G = Z and j j ≤ 2 n 1 A = 1; 3; 3 ;:::; 3 − . It is easy to construct similar examples of sets with large sumset, but ratherf harder to findg examples with A + A small. Let us think more carefully about this problem in the special case G = Z. Proposition 1 Let A Z have size n. Then A + A 2n 1, with equality if and only if A is an arithmetic progression⊆ of length n. j j ≥ − Proof. Write A = a1; : : : ; an where a1 < a2 < < an. Then we have f g ··· a1 + a1 < a1 + a2 < < a1 + an < a2 + an < < an + an; ··· ··· which amounts to an exhibition of 2n 1 distinct elements of A + A. There are other ways of exhibiting 2n 1 distinct elements− of A + A; for any 1 i n we have − ≤ ≤ a1 + a1 < < a1 + ai < < ai + ai < < ai + an < < an + an: ··· ··· ··· ··· If A + A = 2n 1, however, these two orderings must coincide exactly for any i. Thus j j − in particular we have a2 + ai = a1 + ai+1, and it is easy to see that this forces A to be an arithmetic progression. 1Trinity College, Cambridge CB2 1TQ. I would be most grateful to receive comments and corrections, which may be emailed to me at [email protected]. 1 It is easy to exhibit rather less structured sets for which A + A 4n, say; simply take an arithmetic progression of length 2n and select any n elementsj fromj ≤ it. Such sets are still covered rather economically by an arithmetic progression. There are, however, examples of sets with small sumset which are not of this form. Let x0; x1; : : : ; xd Z and let m1; : : : ; md be positive integers. The set 2 d ( ) P = x0 + λjxj 0 λj mj 1 X ≤ ≤ − j=1 is said to be a d-dimensional progression. We say that P is proper if P = m1m2 : : : md, that is to say if all of the sums comprising P are distinct. If P is proper thenj j it is easy to confirm that P + P 2d P . j j ≤ j j Freiman's beautiful and deep theorem states that these are essentially the only examples of subsets of Z with small sumset. The qualitative form of his result is the following. Theorem 2 (Freiman,[4]) Let A Z have cardinality n, and suppose that A+A C A . Then A is contained in a proper d-dimensional⊆ progression P of size at mostj Kn,j ≤ wherej jd and K depend only on C. 1.2 Pl¨unnecke's Inequalities If k and l are positive integers then we may generalise the concept of a sumset as follows. Let A be a subset of an abelian group G, and write kA lA for the set consisting of all elements − of G of the form a1 + + ak a10 al0 (we will also use such notations as kA + lB, whose meanings should··· be obvious).− − · · · − Theorem 3 (Pl¨unnecke { Ruzsa) Suppose that A + A C A . Then kA lA Ck+l A for any k; l. j j ≤ j j j − j ≤ j j Pl¨unnecke proved some results which are at least in a similar spirit to this in several papers and in the monograph [12]. As well as being in German, these papers suffer from a surfeit of notation and I was unable to make much headway with them. Thus the worker in this area should be grateful to Ruzsa [15] for rediscovering and simplifying Pl¨unnecke's work, so that we can give the polished treatment that follows. We will deduce Pl¨unnecke's inequalities from a rather different looking result. Before stating this we need to make some definitions. A Pl¨unnecke Graph of level h is a directed graphG = (V (G);E(G)) on some vertex set V0 V1 Vh satisfying the following properties: [ [···[ (i) All edges in E(G) are of the form (v; v0) where v Vi and v0 Vi+1 for some 0 i h 1; 2 2 ≤ ≤ − (ii) (Forward splitting of paths) Let 0 i h 2 and suppose that u Vi, v Vi+1 and ≤ ≤ − 2 2 w1; : : : ; wk Vi+2 are such that (u; v) and all of the (v; wj) are edges of G. Then there are 2 2 distinct v1; : : : ; vk Vi+1 such that all of the (u; vj) and the (vj; wj) are edges of G. 2 (iii) (Backward splitting of paths) Let 0 i h 2 and suppose that u1; : : : ; uk Vi, ≤ ≤ − 2 v Vi+1 and w Vi+2 are such that (v; w) and all of the (uj; v) are edges of G. Then there 2 2 are distinct v1; : : : ; vk Vi+1 such that all of the (uj; vj) and the (vj; w) are edges of G. 2 It goes without saying that the reader is advised to draw a picture representing properties (ii) and (iii), whereupon their meaning will be clarified enourmously. Now if X V0 we write ⊆ imi(X) for the set of all vertices in Vi which can be reached by a path starting from some x X. The ith magnification ratio of G, Di(G), is defined by 2 imi(X) Di(G) = inf j j: X V0;X= X ⊆ 6 ; j j Proposition 4 (Pl¨unnecke) Let G be a Pl¨unnecke graph of level h 2. Then we have the inequalities ≥ 1=2 1=3 1=h D1 D D D : ≥ 2 ≥ 3 ≥ h The deduction of Theorem 3 from Proposition 4 is a relatively simple matter which, further- more, furnishes us with an example of a Pl¨unnecke graph which it may be useful to have in mind. The key step is the following. Proposition 5 Let A; B be subsets of an abelian group with A + hB C A . Then, for h =h j j ≤ j j any h0 h, there is a set A0 A with A0 + h0B C 0 A0 . ≥ ⊆ j j ≤ j j Proof. Define a directed graph as follows. Set Vi = A + iB, and join v Vi to v0 Vi+1 2 2 precisely if v0 v B. It is very easy to check that the graph so defined is Pl¨unnecke; we − 2 denote it by Pl¨un(A; B). The hth magnification ratio, Dh, is at most C because imh(Z) imh(A) A + hB inf j j C: Z A Z ≤ A ≤ A ≤ ⊆ j j j j j j h0=h It follows from Proposition 4 that Dh0 C for any h0 h, which is equivalent to the statement of the proposition. ≤ ≥ It follows immediately, taking h = 1, that if A+A C A then kA Ck A for any k 2. To deduce the full strength of Theorem 3 wej needj a ≤ furtherj j smallj lemma.j ≤ j j ≥ Lemma 6 Let U; V; W be subsets of an abelian group. Then we have U V W U + V U + W : j jj − j ≤ j jj j Proof. For any d V W fix v(d) V , w(d) W with v(d) w(d) = d. We define a map from U (V W )2 to (U−+ V ) (U +2W ) by sending2 (u; d) to (−u + v(d); u + w(d)). It is easy × − × 3 to check that this is injective. We may now complete the proof of Theorem 3. Suppose that A + A C A , and suppose j j ≤ j j without loss of generality that l k. We may apply Proposition 5 twice to get sets A00 ≥ ⊆ A0 A satisfying ⊆ k A0 + kA C A0 j j ≤ j j and l A00 + lA C A00 : j j ≤ j j Lemma 6 then gives A00 kA lA A00 + kA A00 + lA j jj − j ≤ j jj j A0 + kA A00 + lA ≤ j k+l jj j C A0 A00 ≤ k+lj jj j C A A00 : ≤ j jj j Cancelling the common factor of A00 completes the deduction of Theorem 3 from Proposition 4. j j We turn now to the proof of Proposition 4. This proposition certainly implies that if Dh 1 ≥ then Di 1 for all i h. It turns out that, with rather more effort, one can reverse this implication.≥ Therefore≤ we begin by proving Proposition 7 Let G be a Pl¨unnecke graph of level h, and suppose that Dh 1. Then ≥ Di 1 for all i. ≥ Proof. Let V (G) = V0 Vh. We will show that if Dh 1 then there are V0 vertex- [···[ ≥ j j disjoint paths from V0 to Vh, from which it will be immediate that Di 1 for all i.
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