Now we are ready to check that the presentationZ(M,Λ,ra)satisfies conditions (Z1), (Z2), (Z3) from Section 3.
Here we are going to use notation from Section 3 again, as in Section 8.
Let R0=S0 be the set Z(M,ra) of all relations in Z(M,Λ,ra) except for the hub, and let S1/2 = {Λ(0)}, so Z(M,Λ,ra) = R1/2. We let Y = K, so all letters from R∪A are 0-letters.
Condition (Z1.1) holds by an easy inspection. Condition (Z1.2) holds by Propo-sition 8.2.
The length of Λ is (2n+ 3)N > n, so (Z2.1) holds. The condition (Z2.2) is obviously true.
The next lemma gives us (Z2.3).
Lemma 9.8. — Assume thatv1w1 andv2w2 are cyclic permutations of Λ(0)±1 and |v1| ≥ ε|Λ(0)|. Then if u1v1 =v2u2 holds modulo S0 for some 0-words u1, u2, then we have u2w1 = w2u1 modulo S0.
Proof. — Suppose that u1v1 = v2u2 modulo S0. Then there exists a van Kam-pen diagram Γ over S0 with boundary p1q1p−21q−21 such that φ(p1)≡u1, φ(p2)≡u2, φ(q1)≡v1, φ(q2)≡v2 (see Figure 6). TheK-bands ofΓ starting on q1 end onq2 since φ(q1) is a linear word and u1,u2 do not contain letters from K ∪K−1. The labels of the start and end edges of each of these bands are equal. Hencev2≡φ(q2)≡φ(q1)≡ v1 ≡ v. Since |v| > 0, and Λ(0) and Λ(0)−1 do not have common letters, v cannot be a subword of both Λ(0) and Λ(0)−1. Without loss of generality assume that v is a subword of Λ(0).
Recall that for every j = 1, ...,N, we denote Λj(0) to be the subword of Λ(0) starting with λ(j) and ending with λ(j+1)( as usual “N+1” here is 1).
By Lemma 9.6 there exists a word h over M such that v·h =v, u1 =L(h,z1), u2 = R(h,z2) modulo Z(M,ra) for some z1,z2. Hence we can assume that u1 = L(h,z1), u2 =R(h,z2) in the free group and that Γ= T(v,h). Since T(v,h) is a dia-gram overZ(M,a), Γ does not contain Burnside ra-cells.
Since v ≡ φ(q1) ≡ φ(q2) is a subword of Λ(0), we can attach to Γ two hubs Π1 and Π2 along the paths q1 and q2, and consider the resulting diagram ∆ with boundary label w2u1w−11u−21. We need to show that ∆ can be replaced by a diagram over S0 with the same boundary label.
Since |v1| ≥ ε|Λ(0)| and N > n > 3/ε, we conclude that v1 contains Λj(0) for some j > 1.
Let q1 be the subpath of q1 such that φ(q1) ≡ Λj(0). Since Λ is linear, every K-band starting on q1 ends on q2. Let T1 be the λ(j)-band and T2 be the λ(j+1) -band starting on q1. Then T1 ends on the edge of q2 labeled by λ(j,0) and T2 ends on the edge of q2 labeled by λ(j+1,0).
The bands T1, T2, the path q1 and the path q2 bound a subdiagram Γ of Γ which is a trapezium. By Lemma 7.5, Γ = T(Λj(0),h) and Λj(0)·h = Λj(0). Let t be the subpath of ∂(Γ)which is a side of T1.
Consider the subdiagram ∆ of ∆ consisting of Π1, Π2 and t. Since ∆\∆ con-sists of 0-cells, it suffices to show that the boundary label of ∆ is equal to 1 mod-ulo S0.
Thus we need to show that φ(t)=L(h,λ(j)) commutes with Λj(0)λ(j+1,0)−1Λj+1(0)λ(j+2,0)−1...Λj−1(0)λ(j,0)−1. (26)
By Proposition 6.16, for every i =1, ...,N we have Λi(0)·h=Λi(0). The trapezium T(Λi(0),h) gives us the equality
Λi(0)−1L(h,λ(i))Λi(0)=R(h,λ(i+1))
=λ(i+1,0)−1(L(h,λ(i+1))λ(i+1,0), i =1, ...,N modulo S0. Hence
λ(i+1,0)Λi(0)−1L(h,λ(i))Λi(0)λ(i+1,0)−1=L(h,λ(i+1)), i=1, ...,N.
Hence if we conjugate L(h,λ(i)) by the word (26), we get L(h,λ(i)) as required.
As in Section 3, for every cyclically reduced essential element g ∈ Hkra we de-fine 0(g) as the maximal subgroup of Hra which contains g in its normalizer. For an arbitrary essential element g of the form vuv−1 where u is cyclically reduced, we de-fine 0(g) = v0(u)v−1. By Lemma 3.7, 0(g) is well defined (does not depend on the presentationg =vuv−1).
The following lemma will imply both (Z3.1) and (Z3.2).
Lemma 9.9. — Let w be an essential element represented by a cyclically minimal in rank 1/2 word W, let x = 1 be a 0-element in Hkra such that w−4xw4 is a 0-element. Then the following two statements hold.
(i) There exists a 0-element u such that wu−1 commutes with every element of 0(w). (ii)x ∈0(w).
Proof. — We divide the proof into several steps.
1. Let P1 be a 0-word representing x. By Lemma 3.4 every cyclic shift of W is a cyclically K-reduced word. Therefore we can assume that W starts with a letter z∈K∪K−1 (by Lemma 3.7).
By Lemma 3.6, (Wz)−1P1Wz is equal in Hkra to a 0-word P. By Lemma 9.5 Wz=CBzD in Hkra where B does not contain R-letters and starts with a K-letter z such that Cz = zC, zD = Dz modulo Z(M,a) where D ≡ L(f,z), C ≡L(f,z) for some words f,f over M.
In particular since Wz = CBzD =CBDz, we have W= CBD in Hkra. Since DCz =zDC, passing if necessary to a cyclic shift BDC of W, we can assume that C is empty, so z =z, and replace W by BD where Dz=zD modulo Z(M,a). The wordB is equal to zB1Uwhere B1 is either empty or ends with a letter z∈K∪K−1, Uis a word over A.
2. By Lemma 3.6, we have the following equalities in Hkra: (zB1UDzB1Uz)−1P1(zB1UDzB1Uz)=P2
(27)
(zB1UDzB1UDzB1Uz)−1P1(zB1UDzB1UDzB1Uz)=P3
(28)
whereP2 and P3 are 0-words.
By Lemma 9.7 (which can be applied because zB1UDzB1Uz is reduced since this word is equal to a subword of the same length of the reduced word W3) we can assume that equality (27) holds moduloZ(M,a)(replacing P1 and P2 by equal in Hkra
words if necessary), Bz is a reduced admissible word. By Lemma 3.6, since Dz =zD, we have that(zB1Uz)−1P1(zB1Uz)is a0-word moduloZ(M,a). Hence by Lemma 9.6 P1=L(h,z) modulo Z(M,a) and
Bz·h=Bz. (29)
3. Now consider a diagram∆ overZ(M,a)corresponding to equality (27),∂(∆)
= p1q1p−21q2−1 where φ(p1) ≡ P1, φ(p2) ≡ P2, φ(q1) ≡ BDBz ≡ φ(q2). Then q1 = s1t1s1, where φ(s1)≡B, φ(t1) ≡D, φ(s1)≡Bz. Consider the maximal z-band T1 in
∆starting on the first edge ofs1. As usual, we can assume that p2 is a side of az-band T2. The bands T1 and T2 bound a subdiagram ∆1 of ∆ which is a trapezium. By Lemma 7.5, ∆1=T(Bz,h1) where h1 is the history of T1.
Consider the natural homomorphism ψ from Hkra to the free group freely gen-erated by M. This homomorphism takes K and A to 1, and takes each letter from R to the corresponding rule from M. It is clear that this homomorphism is correctly defined because killing all letters from K∪A in every relation from Z(M,a) and re-placing letters from R by the corresponding rules from M gives us a trivial relation 1= 1. Since D = L(f,z), ψ(D)= f. Applying this homomorphism to the bound-ary label of the subdiagram of∆ bounded by p1 and a side of T1, we get h1=f−1hf. Hence we have
Bz·f−1hf =Bz. (30)
4. Now let ∆1 be a van Kampen diagram corresponding to equality (28). Ap-plying Lemma 9.6 to the subdiagram∆2 of∆1 bounded by two z-bands T1, T2 cor-responding to the two last distinguished occurrences of z in zB1UDzB1UDzB1Uz, we obtain zB1Uz ·h2 = zB1Uz, i.e. Bz ·h2 = Bz where h2 is the history of the z-band T1. Applying the homomorphism ψ to the boundary label of the subdiagram
of∆2=∆1\∆2 we obtain the equality ψ(D)−2hψ(D)2h−21=f−2hf2h−21 modulo Burn-side relations. Hence f−2hf2h2−1 =1 modulo Burnside relations. We conclude that
Bz·h2 =Bz (31)
whereh2=f−2hf2 modulo Burnside relations.
Let W1=Bz, h1=f−1hf, g =f, b=h2. ThenW1·h1=W1 by (30),W1·gh1g−1 = W1 by (29) and W1·b=W1 by (31) where b=g−1h1g modulo Burnside relations.
Suppose that Bz contains an accepted subwordB. ThenB·h is a cyclic shift C ofΛ(0)for some word h overM. Then for some admissible subwordB ofBandB, B·h =C where CC ≡C, |C| ≤ 1. By Lemma 7.5 B =UCV in Hkra where U andVare 0-words. HenceB =U(C)−1Vin rank 1/2. Since |U(C)−1V|<|UCV|, and B is a subword of W, we get a contradiction with the assumption that W is cyclically minimal in rank 1/2.
Thus all conditions of Proposition 6.17 hold. By that proposition and by Re-mark 6.18, we can conclude that for every integers there exists an word bs+1 over M which is equal to gsh1g−s =f−s−1hfs+1 modulo Burnside relations and such that
Bz·bs+1=Bz.
5. For arbitrary integer s consider the trapezia Ts = T(Bz,bs). Let ∂(Ts) = p1q1p−12 q−12 , where φ(p1)=L(bs,z), φ(p2)=R(bs,z), φ(qi)=Bz, i=1,2.
Considering the subdiagram of Ts obtained by removing the last z-band (whose side is p2), we get the equality
L(bs,z)BL(bs,z)−1=B
moduloZ(M,a). ThusBcommutes with L(bs,z)modulo Z(M,a). Notice thatb0=h.
HenceBcommutes withL(f,z)−sL(h,z)L(f,z)s moduloZ(M,ra). Recall thatL(f,z)
≡ D, L(h,z) ≡ P1 ≡ L(b0,z). Hence (D)−sx(D)s commutes with B for every inte-ger s.
Therefore for every non-negative integer s, we have the following equalities in Hkra:
W−sP1Ws=
(BD)−sP1(BD)s=(D)−1B−1...(D)−1B−1(D)−1(B−1P1B)DBD...BD= (D)−1B−1...(D)−1B−1((D)−1P1D)BD...BD=
(D)−1B−1...B−1((D)−2P1(D)2)B...BD=...
(D)−sP1(D)s (32)
(we underline the parts of the words which are changing). Similar equalities hold for negatives. Therefore for every integers, w−sxws is a0-element inHkra. Hencex∈0(w)
(since the subgroup generated by w−sxws, s = 0,±1, ..., consists of 0-elements and is normalized by w).
6. Let us prove that Bcommutes with every elementy∈0(w)modulo Z(M,ra). Let Py be a word in R∪A representing y. Since y ∈ 0(w), W−2PyW2 is equal mod-ulo Z(M,ra) to a 0-word. By Lemma 3.6, (BDz)−1Py(BDz) is a 0-word (modulo Z(M,ra)). Since Dz = zD modulo Z(M,ra), we have that (BzD)−1Py(BzD) is a 0-word moduloZ(M,ra). By Lemma 3.6,(Bz)−1Py(Bz)is a0-wordPy moduloZ(M,ra). Considering a van Kampen diagram for this equality, we deduce (by a usual argu-ment) that Py = L(hy,z), Py = R(hy,z) for some words hy,hy over M. Applying the homomorphism ψ to the equality
(Bz)−1L(hy,z)(Bz)=R(hy,z)
we get that hy =hy modulo Burnside relations. The last equality implies B−1PyB=B−1L(hy,z)B=zR(hy,z)z−1 =L(hy,z)=Py
(33)
modulo Z(M,ra) (by Lemma 7.1). Hence B commutes with Py modulo Z(M,ra), as required.
7. Now let y ∈0(w) be represented by a word Py. Let us prove by induction on l that (D)lPy(D)−l belongs to 0(w) for every l ≥ 0. This is clearly true for l = 0.
Suppose that it is true for some l ≥0. Then
(D)l+1Py(D)−l−1=(D)(D)lPy(D)−l(D)−1
=B−1(BD)(D)lPy(D)−l(BD)−1B. (34)
Since(D)lPy(D)−l belongs to0(w)=0(BD), we have that(BD)(D)lPy(D)−l(BD)−1 belongs to 0(w) (by the definition of 0(w)). Since B commutes with every element of 0(w), (34) implies that
(D)l+1Py(D)−l−1=(BD)(D)lPy(D)−l(BD)−1 ∈0(w).
Similarly one can prove that (D)lPy(D)−l belongs to 0(w)for all l < 0.
8. Let u be the element ofHkra represented by D. Then clearlyu is a0-element.
Let w be the element represented by W(D)−1. We need to prove that w commutes with y, that is, W(D)−1 commutes withPy modulo Z(M,ra), or, equivalently,
(BD)−1Py(BD)=(D)−1Py(D)
modulo Z(M,ra). This has been proved already in (33).
This completes verification of properties (Z1), (Z2), (Z3). By Proposition 3.19, we have the following statement.
Proposition 9.10. — There exists a graded presentation Rkra containing Z(M,ra) of the factor groupHkra(∞)ofHkra over the subgroup generated by alln-th powers of elements ofHkra, such that every relation of Rkra of rank ≥ 1 has the form un =1 for some word u, and every minimal diagram over Rkra satisfies property A from Section 2.4, and all the lemmas from Section 3.
10. Proof of Theorem 1.3
Lemma 10.1. — For every word w(x1, ...,xm), if w(a1(κ(1)), ...,am(κ(1))) = 1 in Hkra(∞), then w(a1, ...,am)=1 in A(κ(1)).
Proof. — Let ∆ be a g-reduced diagram over Rkra with boundary label w(a1(κ(1)), ...,am(κ(1))). Suppose that ∆ contains cells of rank > 0. By Prop-osition 9.10 ∆ is an A-map. By Lemma 2.5 ∆ contains a cell Π of rank > 0 and a contiguity subdiagram of Π to ∂(∆). Since non-0 letters in presentation Rkra are letters from K, by definition of contiguity subdiagrams from Section 2, ∂(∆) contains a K-edge, a contradiction.
Hence ∆ does not contain cells of rank > 0. Therefore ∆ is a diagram over Hkra. So we can assume that ∆ is a g-reduced diagram over Hkra. By Lemma 9.2 we can assume that ∆ does not contain K-annuli. Hence ∆ does not contain K-edges.
Therefore∆ is a diagram over Rra where R is the set of non-0 letters.
Suppose that ∆ contains a cell of rank > 0. Then by Proposition 8.2 and Lem-ma 2.6 ∆ contains a cell Π of rank > 0 and a contiguity subdiagram of Π to ∂(∆). Hence ∂(∆) contains R-edges, a contradiction. Therefore ∆ does not contain cells of rank > 0. Hence ∆ is a diagram over Hra. By Lemma 7.3, part (3), ∆ does not contain R-annuli. Hence ∆ does not contain R-edges. This means that all cells of ∆ are a-cells. Therefore ∆ is a diagram over Ha.
Since A(κ(1)) is a retract of Ha, we obtain that w(a1(κ(1)), ...,am(κ(1))) is equal to 1 modulo R(κ(1))-relations and Burnside relations, as required.
Given the Burnside group B(m,n) generated by a1, ...,am, we have, by Lem-ma 5.2, a homomorphism φ:B(m,n)→H such that φ(ai)=ai(κ(1)), i=1, ...,m.
Assume that φ(w(a1, ...,am)) ∈ Hn. Then w(a1(κ(1)), ...am(κ(1))) = 1 in Hkra(∞)=H(n)=H/Hn by Proposition 9.10. Setting R(κ(1))=∅, we have from Lemma 10.1 that w = 1 in A(κ(1)) = B(a1, ...,am;n). Hence φ is an embedding, and moreover, the intersection of φ(B(m,n))∩Hn is trivial. This proves Statement (1) of Theorem 1.3.
Let now L be an arbitrary normal subgroup of B(m,n) (the latter can now be identified with its image inH/Hn). To prove Statement (2) of Theorem 1.3, we have to show that L = LH/Hn ∩B(m,n) = L. Let us define R(κ(1)) to be the set of all words in a1(κ(1)), ...,am(κ(1)) representing elements of L. The new group Hkra(∞) (whose construction depends now on the new choice of R(κ(1))) is a homomorphic image of H/Hn by Proposition 9.10. Therefore it suffices to prove that every word w(a1(κ(1)), ...,am(κ(1))) that is equal to 1 in Hkra(∞), is also trivial in B(m,n)/L ∼= A(κ(1)). But again, this claim is a part of Lemma 10.1.
The proof of Theorem 1.3 is complete.
REFERENCES
1. S. I. ADIAN, Random walks on free periodic groups, Izv. Akad. Nauk SSSR, Ser. Mat.,46(6) (1982), 1139–1149.
2. S. I. ADIAN, The Burnside problem and identities in groups, Ergebnisse der Mathematik und ihrer Grenzgebiete [Results in Mathematics and Related Areas], 95, 311 pp. Berlin-New York: Springer, 1979.
3. S. I. ADIAN, Periodic products of groups. Number theory, mathematical analysis and their applications, Trudy Mat.
Inst. Steklov,142 (1976), 3–21, 268.
4. S. BANACH and A. TARSKI, Sur la d´ecomposition de ensembles de points an parties respectivement congruentes.
Fund. math.,6 (1924), 244–277.
5. J. C. BIRGET, A. Yu. OL’SHANSKII, E. RIPSand M. V. SAPIR, Isoperimetric functions of groups and computational complexity of the word problem, Ann. Math.,156(2) (2002), 467–518.
6. M. G. BRINand C. C. SQUIER, Groups of piecewise linear homeomorphisms of the real line, Invent. Math.,79(3) (1985), 485–498.
7. J. W. CANNON, W. J. FLOYD and W. R. PARRY, Introductory notes on Richard Thompson’s groups. L’Enseignement Math´ematique (2), 42(3–4) (1996), 215–256.
8. A. H. CLIFFORD and G. B. PRESTON, The algebraic theory of semigroups, Vol. I, Mathematical Surveys, No. 7, Provi-dence, R.I.: American Mathematical Society, 1961.
9. J. M. COHEN, Cogrowth and amenability of discrete groups, J. Funct. Anal.,48(3) (1982), 301–309.
10. M. M. DAY, Amenable semigroups, Illinois J. Math., 1 (1957), 509–544.
11. E. FØLNER, On groups with full Banach mean value, Math. Scand.,3 (1955), 243–254.
12. Open problems in infinite-dimensional topology, ed. by Ross Geoghegan, The Proceedings of the 1979 Topology Conference (Ohio Univ., Athens, Ohio, 1979), Topology Proc.,4(1) (1979), 287–338 (1980).
13. F. P. GREENLEAF, Invariant means on topological groups and their applications, New York: Van Nostrand Reinhold, 1969.
14. R. I. GRIGORCHUK, Symmetrical random walks on discrete groups, Multicomponent random systems, Adv. Probab.
Related Topics,6 (1980), 285–325, New York: Dekker
15. R. I. GRIGORCHUK, An example of a finitely presented amenable group that does not belong to the class EG, Mat. Sb.,189(1) (1998), 79–100.
16. F. HAUSDORFF, Grundz¨uge der Mengenlehre, Leipzig, 1914.
17. S. V. IVANOV, On HNN-extensions in the class of groups of a large odd exponent, Preprint, 2002.
18. S. V. IVANOV and A. Y. OL’SHANSKII, Hyperbolic groups and their quotients of bounded exponents, Trans. Amer.
Math. Soc.,348(6) (1996), 2091–2138.
19. H. KESTEN, Full Banach mean values on countable groups, Math. Scand.,7 (1959), 146–156.
20. H. KESTEN, Symmetric random walks on groups, Trans. Amer. Math. Soc.,92 (1959), 336–354.
21. O. G. KHARLAMPOVICH and M. V. SAPIR, Algorithmic problems in varieties, Internat. J. Algebra Comput., 5(4–5) (1995), 379–602.
22. Kourovka Notebook, Unsolved Problems in Group Theory, 8th edition, Novosibirsk, 1982.
23. R. LYNDONand P. SCHUPP, Combinatorial group theory, Springer, 1977.
24. K. V. MIKHAJLOVSKII, Some generalizations of the HNN-construction in the periodic case, #1063-B94, VINITI, Moscow, 1994, 59 pp.
25. J. VON NEUMANN, Zur allgemeinen Theorie des Masses, Fund. math.,13 (1929), 73–116.
26. P. S. NOVIKOV and S. I. ADIAN, Infinite periodic groups, I, II, III (Russian), Izv. Akad. Nauk SSSR Ser. Mat., 32 (1968), 212–244, 251–524, 709–731.
27. A. Yu. OL’SHANSKII, An infinite simple torsion-free Noetherian group, Izv. Akad. Nauk SSSR Ser. Mat.,43(6) (1979), 1328–1393.
28. A. Yu. OL’SHANSKII, An infinite group with subgroups of prime order, Izvestia Akad. Nauk SSSR, Ser. Mat.,44(2) (1980), 309–321.
29. A. Yu. OL’SHANSKII, On the question of the existence of an invariant mean on a group (Russian), Uspekhi Mat.
Nauk,35(4(214)) (1980), 199–200.
30. A. Yu. OL’SHANSKII, The geometry of defining relations in groups, Moscow: Nauka, 1989.
31. A. Yu. OL’SHANSKII, The SQ-universality of hyperbolic groups, Mat. Sb.,186(8) (1995), 119–132.
32. A. Yu. OL’SHANSKII, On distortion of subgroups in finitely presented groups, Mat. Sb.,188(11) (1997), 51–98.
33. A. Yu. OL’SHANSKIIand M. V. SAPIR, Embeddings of relatively free groups into finitely presented groups, Contemp.
Math.,264 (2000), 23–47.
34. A. Yu. OL’SHANSKII and M. V. SAPIR, Length and Area Functions on Groups and Quasi-Isometric Higman Em-beddings, J. Algebra and Comp.,11(2) (2001), 137–170.
35. M. V. SAPIR, Problems of Burnside type and the finite basis property in varieties of semigroups, Izv. Akad. Nauk.
SSSR., Ser. Mat.,51(2) (1987), 319–340.
36. M. V. Sapir, J. C. BIRGET and E. RIPS, Isoperimetric and isodiametric functions of groups, 1997, Ann. Math., 156(2) (2002), 345–466.
37. W. SPECHT, Zur Theorie der messbaren Gruppen, Math. Z.,74 (1960), 325–366.
38. J. TITS, Free subgroups of linear groups, J. Algebra, (20) (1972), 250–270.
39. A. M. VERSHIK, Comments to papers by J. von Neumann, in “J. von Neumann, Selected works in functional analysis”, Moscow: Nauka, v1 (1987), 357–376.
properties (M1), (M2), (M3) 54
z−,z+ 108 Z(M) 114 Z(M,Λ) 114
Z(M,a) 142 Z(M,ra) 151 Z(M,Λ,ra) 151
A. Yu. O.
Department of Mathematics Vanderbilt University
1326 Stevenson Center Nashville, TN 37240 USA
alexander.olshanskiy@vanderbilt.edu http://www.math.vanderbilt.edu/∼olsh and
Department of Higher Algebra, MEHMAT Moscow State University
Moscow, Russia
olshan@shabol.math.msu.su M. V. S.
Department of Mathematics Vanderbilt University 1326 Stevenson Center Nashville, TN 37240 USA
http://www.math.vanderbilt.edu/∼msapir
Manuscrit reçu le 8 février 2001.