New rewriting system for the braid group B 4


 Camron Brooks
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1 Leonid Bokut 1 and Andrei Vesnin 1,2 1 Sobolev Institute of Mathematics, Novosibirsk , Russia 2 School of Mathematical Sciences, Seoul National University, Seoul , Korea To 60th birthday of a pioneer of Gröbner bases Prof. B. Buchberger Abstract Using presentations of the braid groups B 3 and B 4 as towers of HNN extensions of the free group of rank 2, we obtain normal forms, Gröbner bases and rewriting systems for these groups. KEYWORDS: braid group, normal form, Gröbner basis 1. Introduction In this note we obtain Gröbner bases as well as rewriting systems for braid groups B 3 and B 4. Braid groups are subject of the intensive studding in group theory and lowdimensional topology. We refer to (Birman 1974) for the basic properties of braid groups. It was shown by P. Dehornoy that braid groups are leftorderable (Dehornoy 2000), and different kinds of normal forms and rewriting systems for braid groups can be found in (Birman et al. 1998, Elrifai Morton 1994, Garside 1969, Garber et al. 2002, Hermiller Meier 1999, Markov 1945, Thurston 1992). Our interest in Gröbner bases for braid groups is motivated by the close relation between noncommutative Gröbner bases and rewriting systems for semigroups. This relation is a kind of a folklore, and was fully described, for example, in (Heyworth 2000). As an introduction to string rewriting we refer to (Book Otto 1993). The basis facts about Gröbner bases will be presented in Section 2. One of aims of this note is to develop and to demonstrate a method of constructing of Gröbner bases, normal forms, and rewriting systems for groups, presented as towers of HNN extensions of free groups. We realize this method for braid groups B 3 and B 4. Authors were supported in part by the Russian Foundation for Basic Research (grants and ).
2 In Section 3, applying the MagnusMoldovanski rewriting procedure (see Lyndon Schupp (1977)) for B 3 and B 4 (which is 3relator group rather than to 1relator classical case) we obtain presentations of these groups as towers of HNN extensions of the free group of rank 2. Obtained presentations are closely related to the presentations of commutators B 3 and B 4 which have been find by E. Gorin and V. Lin (see Gorin Lin (1969)) using the ReidemeisterShreier method. So, we will refer to the generators as GorinLin generators. These presentations lead to the standard normal forms in B 3 and B 4, and to the standard rewriting systems for them in the sense of (Bokut 1966, 1967, 1980). We show it in Section 4, using the Gröbner Shirshov bases technic unlike of a grouptheoretic technic of (Bokut 1966, 1967). More precisely, we give the Gröbner Shirshov basis of B 4 (and as a corollary, for B 3 ) in GorinLin generators and relative to an appropriate order of group words, the tower order. This order was implicitly used in (Bokut 1966, 1967). The applications of similar technique to Coxeter groups and Novikov and Boon groups can be found in (Bokut Shiao 2001, 2002). From the normal form, it readily follows some known properties of B 4 proved in the above mentioned paper by E. Gorin and V. Lin, in particular, the presentation of the commutator group B 4 and its description as the semidirect product of two free groups of rank two. Moreover, the relation of obtained presentations of braid groups with cyclically presented groups, such as Sieradski groups and Fibonacci groups is discussed. This work was done during the first author visit to the KIAS, Seoul. We thank Dr. SangJin Lee for very helpful discussions. 2. NonCommutative Gröbner bases In this section we recall some facts about noncommutative Gröbner bases which are also known in the literature as Gröbner Shirshov bases (see, for example, Ufnarovski (1998)). Let X be a linearly ordered set, k a field, k X the free associative algebra over X and k. On the set X of words we impose a well order > that is compatible with the concatenations of words. For example, it may be the deglex order (compare two words first by degrees and then lexicographically) or the tower order below. Any polynomial f k X has the leading word f relative to >. We say that f is monic if f occurs in f with coefficient 1. By a composition of intersection (f, g) w of two monic polynomials relative to some word w, such that w = fb = aḡ, deg( f) + deg(ḡ) > deg(w), one means the following polynomial (f, g) w = fb ag. By composition of including (f, g) w of two monic polynomials, where w = f = aḡb, one means the following polynomial (f, g) w = f agb.
3 In the last case the transformation L. Bokut, A. Vesnin f (f, g) w = f agb is called the elimination of the leading word (ELW) of g in f. A composition (f, g) w is called trivial relative to some R k X and w (we write it as (f, g) w 0 mod (R, w) ) if (f, g) w = α i a i t i b i, where α i k, t i R, a i, b i X, and a i t i b i < w. In particular, if (f, g) w goes to zero by the ELW s of R then (f, g) w is trivial relative to R. For two polynomials f 1 and f 2 we write if and only if f 1 f 2 mod (R, w) f 1 f 2 0 mod (R, w). A subset R of k X is called Gröbner Shirshov basis if any composition of polynomials from R is trivial relative to R. By X R, the algebra with generators X and defining relations R, we will mean the factoralgebra of k X by the ideal generated by R. The following lemma goes back to the PoincareBirkgoffWitt theorem, the Diamond Lemma of M.H.A. Newman (Newman 1942), the Composition Lemma of A.I. Shirshov (Shirshov 1962) (see also Bokut (1972, 1976), where this Composition Lemma was formulated explicitly and in a current form), the Buchberger s Theorem (Buchberger (1965), published in Buchberger (1970)), the Diamond Lemma of G. Bergman (Bergman 1978) (this lemma was also known to P.M. Cohn (see, for example, Cohn (1966)) and some historical comments to Chapter Gröbner basis in (Eisenbud 1995)): Composition Diamond Lemma. R is a Gröbner Shirshov basis if and only if the set P BW (R) = {u X u a fb, for any f R} of Rreduced words consists of a linear basis of the algebra X R. The set P BW (R) will be called the PBWbasis of X R relative to a Gröbner Shirshov basis R. If a subset R of k X is not a Gröbner Shirshov basis then one can add to R all non trivial compositions of polynomials of R, and continue this process (infinitely) many times in order to have a Gröbner Shirshov basis R comp that contains R. A Gröbner Shirshov basis R is called reduced if any s R is a linear combination of R \ {s} reduced words. Any ideal of k X has a unique reduced Gröbner Shirshov basis.
4 If R is a set of semigroup relations (that is, polynomials of the form u v, where u, v X ), then any non trivial composition will have the same form. As the result the set R comp consists of semigroup relations too. Let A = smg X R be a semigroup presentation. Then R is a subset of k X and one can find a Gröbner Shirshov basis R comp. The last set does not depend of k, and consists of semigroup relations. We will call R comp to be a Gröbner Shirshov basis of A. It is the same as a Gröbner Shirshov basis of the semigroup algebra ka = X R. The same terminology is valid for any group presentation meaning that we include in this presentation all trivial group relations of the form xx 1 = 1, x 1 x = 1, x X. 3. Presentations of groups B 3 and B 4 In this section we will obtain presentations of braid groups as towers of HNN extensions of the free group of rank two. We start from the consideration of the 3strand braid group B 3 with the following presentation: B 3 = σ 1, σ 2 σ 1 σ 2 σ 1 = σ 2 σ 1 σ 2. (1) Let us denote t = σ 1 and consider y 2 such that σ 2 = y 2 t, i.e. y 2 = σ 2 σ 1 1. Then we have B 3 = y2, t y 2 tty 2 = ty 2 t. Introducing notation y 2(i) = t i y 2 t i for i = 1, 2, we get B 3 = y2, y 2(1), y 2(2), t y 2 y 2(2) = y 2(1), y 2(1) t = ty 2, y 2(2) t = ty 2(1). Eliminating y 2(2) using the first relation, we will obtain: B 3 = y2, y 2(1), t y 2(1) t = ty 2, y 1 2 y 2(1) t = ty 2(1) and finally, B 3 = y2, y 2(1), t y 2(1) t = ty 2, y 2 t = ty 2 y 1 2(1), where we used the first relation to modify the second. Let us rewrite this relation using notations y 2 = t 2 and y 2(1) = t 1 : B 3 = t1, t 2, t t 1 t = tt 2, Thus, we have the following extension: t 2 t = tt 2 t 1 1. (2) t 1, t 2 B 3 = t 1, t 2, t, (3) that can be regarded as an HNNextension (by the conjugation automorphism t) of the free group with two generators. The relation of these generators with the standard generators of B 3 is the following: t 2 = σ 2 σ1 1 and t 1 = σ 1 σ 2 σ1 2. Braids corresponding to t, t 1, and t 2 are presented in the following figure.
5 L. Bokut, A. Vesnin t : t 1 : t 2 : As the result, the kernel of the homomorphism from B 3 to t defined by t t, t 1 1, t 2 1, is the free group t 1, t 2. This kernel coincides with the commutant B 3, that was also shown in (Gorin Lin 1969) by using of the Reidemeister Schreier method. So, we will refer to t, t 1 and t 2 as GorinLin generators of B 3. From the defining relations (2) and their inverses we obtain relations: where we used t 2 t = tt 2 t 1 1, t 1 t 1 = t 1 t 1 2 t 1, t 1 t = tt 2, t 2 t 1 = t 1 t 1, t 2 t = tt 2 t 1 1 t 2 t = t 1 tt 1 1 t 1 1 t 2 t = tt 1 1, t 1 t 1 = t 1 t 1 2 t 1. In the next section we will prove that the following transformations give rise the rewriting system for B 3 (that will follows from the rewriting system for B 4 and embedding B 3 B 4 ): (t 2 ) ±1 t t(t 2 t 1 1 ) ±1, (t 1 ) ±1 t 1 t 1 (t 1 2 t 1 ) ±1, (t 1 ) ±1 t t(t 2 ) ±1, (t 2 ) ±1 t 1 t 1 (t 1 ) ±1. It gives the following normal form for B 3 : t n V (t 1, t 2 ) where n Z and V (t 1, t 2 ) denotes a group word in t 1, t 2. We would like to point out the following relation of the braid group presentation (2) with cyclically presented groups (in sense of Johnson (1980)). Consider the conjugation action of t on the group B 3 = t 1, t 2. Let us denote a 0 = t 2 and a i = t i a 0 t i for i Z. In particular, we get a 1 = t 1 and a 1 = t 2 t 1 1 = a 0 a 1 1. Therefore, for each i we have a i a i+2 = a i+1, and the following group presentation with infinite number of generators naturally arises: G = a i, i Z a i a i+2 = a i+1, i Z For a reader convenience we recall the expression of a i in terms of the Artin generators (1): a i = σ i 1σ 2 σ (i+1) 1. Remark that the truncated version of this group, i.e. G n = a 1,..., a n a i a i+2 = a i+1, i = 1,..., n,
6 where all suffices are taken by mod n, is known as the Sieradski group (see, for example, Cavicchioli et al. (1998)) and is the fundamental group of the nfold cyclic branched covering of the 3sphere, branched over the trefoil knot. Now let us consider the 4strand braid group B 4 with the following presentation: B 4 = σ 1, σ 2, σ 3 σ 2 σ 1 σ 2 = σ 1 σ 2 σ 1, σ 3 σ 2 σ 3 = σ 2 σ 3 σ 2, σ 3 σ 1 = σ 1 σ 3. (4) Let us denote t = σ 1. Consider y 3 such that σ 3 = y 3 t, i.e. y 3 = σ 3 σ 1 1, and y 2 such that σ 2 = y 2 t, i.e. y 2 = σ 2 σ 1 1. Then we have B 4 = y2, y 3, t y 2 tty 2 = ty 2 t, y 3 ty 2 ty 3 = y 2 ty 3 ty 2, y 3 t = ty 3. As well as above, let us introduce y j(i) = t i y j t i for j = 2, 3 and i Z. Then B 4 = y2, y 2(1), y 2(2), y 3, t y 2 y 2(2) = y 2(1), y 3 y 2(1) y 3 = y 2 y 3 y 2(2), y 3 t = ty 3, y 2(1) t = ty 2, y 2(2) t = ty 2(1), where we used that t and y 3 commute. Eliminating y 2(2) using the first defining relation, we will obtain: B 4 = y2, y 2(1), y 3, t y 3 y 2(1) y 3 = y 2 y 3 y2 1 y 2(1), y 3 t = ty 3, y 2(1) t = ty 2, y2 1 y 2(1) t = ty 2(1). Denoting t 1 = y 2(1), t 2 = y 2 and b = y 3 we get and so, B 4 = t1, t 2, t, b bt 1 b = t 2 bt 1 2 t 1, bt = tb, t 1 t = tt 2, t 1 2 t 1 t = tt 1, B 4 = t1, t 2, t, b t 1 b = b 1 t 2 bt 1 2 t 1, bt = tb, t 1 t = tt 2, t 2 t = tt 2 t 1 1, where we used the third relation to modify the forth relation. Denoting a = t 1 bt 1 1, we get B 4 = a, b, t1, t 2, t t 1 t = tt 2, t 2 t = tt 2 t 1 1, bt = tb, (5) at 1 = t 1 b, a = b 1 t 2 bt 1 2. Generators t, t 1, t 2, a, b will be referred to as GorinLin generators of B 4. Using above defining relations we have at = t 1 bt 1 1 t = t 1 btt 1 2 = t 1 tbt 1 2 = tt 2 bt 1 2 = tba. (6) Let us multiply both sides of the relation t 1 b = b 1 t 2 bt 1 2 t 1
7 L. Bokut, A. Vesnin from the left by t, and remark that after that the left part can be modified as tt 1 b = t 1 2 tt 2 b = t 1 2 t 1 tb = t 1 2 t 1 bt = t 1 2 at 1 t, and the right part can be modified as Thus, that gives us tb 1 t 2 bt 1 2 t 1 = b 1 tt 2 bt 1 2 t 1 = b 1 t 1 tbt 1 2 t 1 = b 1 t 1 btt 1 2 t 1 = b 1 t 1 bt 1 1 tt 1 = b 1 at 1 2 tt 2 = b 1 at 1 2 t 1 t. Using this result we have t 1 2 a = b 1 at 1 2, at 2 = t 2 b 1 a. (7) a = b 1 t 2 bt 1 2 bat 2 = t 2 b bt 2 b 1 a = t 2 b bt 2 = t 2 ba 1 b. (8) Now let us conjugate both sides of the obtained relation by t. Then from the left part of the relation we will get tbt 2 t 1 = btt 2 t 1 = bt 1, and from the right part of the relation we will get Hence tt 2 ba 1 bt 1 = t 1 tba 1 t 1 b = t 1 bta 1 t 1 b = t 1 ba 1 tbt 1 b = t 1 ba 1 b 2. Summarizing (5), (6), (7), (8) and (9) we get bt 1 = t 1 ba 1 b 2. (9) Lemma 3.1: The following relations are valid for GorinLin generators of B 4 : t 2 t = tt 2 t 1 1 t 1 t = tt 2, bt = tb, at = tba, at 1 = t 1 b, bt 1 = t 1 ba 1 b 2, at 2 = t 2 b 1 a, bt 2 = t 2 ba 1 b, and for their inverses: t 1 t 1 = t 1 t 1 2 t 1, t 2 t 1 = t 1 t 1, bt 1 = t 1 b, at 1 = t 1 b 1 a, bt 1 1 = t 1 1 a, at 1 1 = t 1 1 a 2 b 1 a, bt 1 2 = t 1 2 ba, at 1 2 = t 1 2 ba 2. Therefore, we have the description of B 4 as a tower of HNN extensions of the free group of rank two: a, b a, b, t 1, t 2 B 4 = a, b, t 1, t 2, t. The correspondence between GorinLin generators and the standard generators of B 4 is the following: t = σ 1, t 1 = σ 1 σ 2 σ 2 1, t 2 = σ 2 σ 1 1, a = σ 1 σ 2 σ 1 1 σ 3 σ 1 2 σ 1 1, b = σ 3 σ 1 1. Remark that t, t 1 and t 2 generate such a subgroup of B 4 with the presentation (2) that gives us B 3. Braids corresponding to t, t 1, t 2, a and b are presented in the following figure.
8 t : t 1 : t 2 : a : b : In the next section we will find a rewriting system for B 4. As the result, a normal form for B 4 is given by expression t n V (t 1, t 2 )W (a, b), where n Z, and V (t 1, t 2 ), W (a, b) are free group words in alphabets {t 1, t 2 } and {a, b}, respectively. The commutant B 4, the kernel of the homomorphism from B 4 to t, defined by t t, t 1 1, t 2 1, a 1, b 1 is the semidirect product of free groups t 1, t 2 and a, b. Here t 1, t 2 is the commutant of B 3. The kernel of the homomorphism of B 4 to B 3, defined by that is by σ 1 σ 1, σ 2 σ 2, σ 3 σ 1, t t, t 1 t 1, t 2 t 2, a 1, b 1, is the free group a, b. Remark that in (Gorin Lin 1969) all these facts were proved using the ReidemeisterShreier method, where t 1 at and b were chosen as the generators of this free group. Now let us point out the connection of relations from Lemma 3.1 with Fibonacci groups F (2, n) studied by many authors. Consider the conjugation action of t 1 on a, b : t 1 1 at 1 = b, t 1 1 bt 1 = ba 1 b 2. Let us denote z 0 = b and z i = t i 1 z 0 t i 1 for i Z. Thus, we get a = z 1 and z 1 = z 0 z 1z 0. 2 Therefore for i Z we have z i+1 z 1 i z 2 i+1 = z i+2, and the following group with infinite number of generators naturally arise: H = z i, i Z z i+1 z 1 i z 2 i+1 = z i+2, i Z. Remark that this group presentation coincides with the presentation of the commutator subgroup of the figureeight knot group obtained in (Burde Zieschang 1985, p. 35) and action of t 1 on a and b corresponds to the presentation of the figureeight knot as a fibred knot (Burde Zieschang 1985, p. 73).
9 L. Bokut, A. Vesnin Similar to (Kim et al. 2000), rewrite the defining relations as (z 1 i z i+1 ) z i+1 = (z 1 i+1 z i+2), i Z and denote x 2i 1 = z i and x 2i = z 1 i z i+1 for i Z. Then the above relations can be rewritten as x 2i = x 1 2i 1 x 2i+1 and x 2i x 2i+1 = x 2i+2. Then H = x2i 1, x 2i, i Z x i x i+1 = x i+2, i Z. For a reader convenience we recall the expression of x i in terms of the GorinLin generators: x 2i 1 = t i 1 bt i 1, x 2i = t i 1 b 1 t 1 1 bt 1 t i 1, i Z. Thus, relations x 2i 1 x 2i = x 2i+1 follow immediately, and relations x 2i x 2i+1 = x 2i+2 follow from the relation bt 1 = t 1 bt 1 b 1 t 1 1 b 2. The truncated version of this group is wellknown as the Fibonacci group: F (2, 2n) = x 1,..., x 2n x i x i+1 = x i+2, i = 1,..., 2n where all indices are taken by mod 2n. For any n 2 the group F (2, 2n) is the fundamental group of a 3dimensional manifold (see Helling et al. (1998)) which can be described as the nfold cyclic branched cover of the 3sphere branched over the figureeight knot. From the above considerations one can easy obtain expressions of generators of Fibonacci groups in terms of Artin generators of B Rewriting systems for B 3 and B 4 With each of the relations of B 4 from Lemma 3.1 we associate another relation in the natural way shown below. Thus, we will get pairs of relations as follows: t 2 t = tt 2 t 1 1 and t 1 2 t = tt 1 t 1 2 (10) t 1 t = tt 2 and t 1 1 t = tt 1 2 (11) bt = tb and b 1 t = tb 1 (12) at = tba and a 1 t = ta 1 b 1 (13) at 1 = t 1 b and a 1 t 1 = t 1 b 1 (14) bt 1 = t 1 ba 1 b 2 and b 1 t 1 = t 1 b 2 ab 1 (15) at 2 = t 2 b 1 a and a 1 t 2 = t 2 a 1 b (16) bt 2 = t 2 ba 1 b and b 1 t 2 = t 2 b 1 ab 1 (17) t 1 t 1 = t 1 t 1 2 t 1 and t 1 1 t 1 = t 1 t 1 1 t 2 (18) t 2 t 1 = t 1 t 1 and t 1 2 t 1 = t 1 t 1 1 (19) bt 1 = t 1 b and b 1 t 1 = t 1 b 1 (20) at 1 = t 1 b 1 a and a 1 t 1 = t 1 a 1 b (21) bt 1 1 = t 1 1 a and b 1 t 1 1 = t 1 1 a 1 (22) at 1 1 = t 1 1 a 2 b 1 a and a 1 t 1 1 = t 1 1 a 1 ba 2 (23) bt 1 2 = t 1 2 ba and b 1 t 1 2 = t 1 2 a 1 b 1 (24) at 1 2 = t 1 2 ba 2 and a 1 t 1 2 = t 1 2 a 2 b 1 (25)
10 Let S be the set of above listed relations (10) (25) together with trivial relations xx 1 = 1, and x 1 x = 1 for x {a, b, t 1, t 2, t}. We will prove that S is the Gröbner Shirshov basis of B 4 relative to the following tower order of words. Let us order group words in a, b by the deglex order. Any group word in a, b, t 2 has a form u = u 0 t ε 1 2 u k t ε k 2 u k+1, where u i a, b, k 0, ε i = ±1. Define wt(u) = (k, u 0, t ε 1 2,..., u k, t ε k 2, u k+1 ). Let us order wt s lexicographically assuming t 1 < t. Let us define the tower order u > tow v if and only if wt(u) > lex wt(v). In the same way we can define the tower order for group words with the extra letter t 1 and then for group words with the extra letter t. Theorem 4.1: The above described set S is the reduced Gröbner Shirshov basis for B 4 in the GorinLin generators relative to the tower order of group words. Proof: To proof the statement we need to check that all compositions (in the sense of Section 2) of pairs of elements of S are trivial. Here we will do it for few of them. For all others similar considerations are used, but we omit them here. Let us consider the composition (10 l ) (16 l ) of an intersection of left relations in (10) and (16) relative to a word w = at 2 t. We have g = t 2 t tt 2 t 1 1 with the leading word ḡ = t 2 t and f = at 2 t 2 b 1 a with the leading word f = at 2. Therefore w = aḡ = ft. Then (10 l ) (16 l ) = (f, g) w = ft ag = (at 2 t 2 b 1 a)t a(t 2 t tt 2 t 1 1 ) = att 2 t 1 1 t 2 b 1 at tbat 2 t 1 1 t 2 b 1 tba tbt 2 b 1 at 1 1 t 2 tb 1 ba tt 2 ba 1 bb 1 at 1 1 t 2 ta tt 2 bt 1 1 tt 2 t 1 1 a tt 2 t 1 1 a tt 2 t 1 1 a 0, where we used relations from the set S for the ELW s of S. Now, let us check that the composition (10 r ) (24 l ) of right relation of (10) and left relation of (24) is trivial with respect to S and w = bt 1 2 t. We have g = t 1 2 t tt 1 t 1 2 with the leading word ḡ = t 1 2 t and f = bt 1 2 t 1 2 ba with the leading word f = bt 1 2. Therefore w = bḡ = ft. Then (10 r ) (24 l ) = (f, g) w = ft bg = (bt 1 2 t 1 2 ba)t b(t 1 2 t tt 1 t 1 2 ) = btt 1 t 1 2 t 1 2 bat tbt 1 t 1 2 t 1 2 btba tt 1 ba 1 b 2 t 1 2 t 1 2 tb 2 a tt 1 ba 1 bt 1 2 ba tt 1 t 1 2 b 2 a tt 1 ba 1 t 1 2 baba tt 1 t 1 2 b 2 a tt 1 bt 1 2 a 2 b 1 baba tt 1 t 1 2 b 2 a tt 1 t 1 2 baa 1 ba tt 1 t 1 2 b 2 a tt 1 t 1 2 b 2 a tt 1 t 1 2 b 2 a 0,
11 L. Bokut, A. Vesnin where we used relations from the set S for the rewriting process. By the similar considerations for other compositions, the statement of the theorem holds. It easy to see, that any s S is a difference of S \ {s} reduced words. It means that S is a reduced Gröbner Shirshov basis. The above Gröbner Shirshov basis of B 4 gives rise to the rewriting system (semithue system) for B 4 that is defined by rules: t ±1 2 t t(t 2 t 1 1 ) ±1, t ±1 1 t tt ±1 2, b ±1 t tb ±1, a ±1 t t(ba) ±1, a ±1 t 1 t 1 b ±1, b ±1 t 1 t 1 (ba 1 b 2 ) ±1, a ±1 t 2 t 2 (b 1 a) ±1, b ±1 t 2 t 2 (ba 1 b) ±1, t ±1 1 t 1 t 1 (t 1 2 t 1 ) ±1, t ±1 2 t 1 t 1 t ±1 1, b ±1 t 1 t 1 b ±1, a ±1 t 1 t 1 (b 1 a) ±1, b ±1 t 1 1 t 1 1 a ±1, a ±1 t 1 1 t 1 1 (a 2 b 1 a) ±1, b ±1 t 1 2 t 1 2 (ba) ±1, a ±1 t 1 2 t 1 2 (ba 2 ) ±1, xx 1 1, x 1 x 1, where x {a, b, t 1, t 2, t}. As a particular case, we get the rewriting system for the braid group B 3. References Bergman, G. (1978), An Algorithmical Criterion for the Solvability of Algebraic Systems of Equations (German), Adv. in Math. 29, Birman, J. (1974), Braid groups and mapping class groups, Vol. 82 of Annals of Math. Studies, Princeton University Press. Birman, J., Ko, K.H. Lee, S.J. (1998), A new approach to the word and conjugacy problem in the braid groups, Adv. Math. 139, Bokut, L. (1966), On a property of the Boone groups, Algebra i Logika Sem. 5(5), Bokut, L. (1967), On a property of the Boone groups. II, Algebra i Logika Sem. 6(1), Bokut, L. (1972), Unsolvability of the word problem, and subalgebras of finitely presented Lie algebras, Izv. Akad. Nauk SSSR Ser. Mat. 36, Bokut, L. (1976), Imbeddings into simple associative algebras, Algebra i Logika 15, , 245. Bokut, L. (1980), Malcev s problem and groups with a normal form, in Stud. Logic Foundations Math., Word problems, II (Conf. on Decision Problems in Algebra, Oxford, 1976), Vol. 95, NorthHolland, AmsterdamNew York, pp With the collaboration of D.J. Collins.
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