Computational aspects of finite p-groups

Size: px

Start display at page:

Download "Computational aspects of finite p-groups"

Adrian Small
5 years ago
Views:

Clayton VIC 3800, Australia 5th 14th November 2016

1 Computational aspects of finite p-groups Heiko Dietrich School of Mathematical Sciences Monash University Clayton VIC 3800, Australia 5th 14th November 2016 International Centre for Theoretical Sciences Bangalore Go to Overview

.. University of Braunschweig (2000-2009) one of the

University of Auckland (2009-2011) work with Magma

(2011-2013) more work with GAP Monash University (since

2 Motivation Hello! Why p-groups? Outline Resources Welcome! And a bit about myself... University of Braunschweig ( ) one of the four GAP centres PhD (on p-groups with maximal class) University of Auckland ( ) work with Magma further research on p-groups University of Trento ( ) more work with GAP Monash University (since 2013) Heiko Dietrich (heiko.dietrich@monash.edu) Computational aspects of finite p-groups ICTS, Bangalore 2016

3 Motivation Hello! Why p-groups? Outline Resources Welcome! In this lecture series we discuss Computational Aspects of Finite p-groups. A finite p-group is a group whose order is a positive power of the prime p. Convention Throughout, p is a prime; unless stated otherwise, all groups and sets are finite. Lecture Material Slides etc will be uploaded at Assumed knowledge Some group theory...

4 Motivation Hello! Why p-groups? Outline Resources Why p-groups?

5 Motivation Hello! Why p-groups? Outline Resources There s an abundant supply of p-groups ord. # ord. # ord. # ord. # ord. # there are p 2n3 /27+O(n 5/3) groups of order p n proved and improved by Higman (1960), Sims (1965), Newman & Seeley (2007) conjecture: almost all groups are p-groups (2-groups) for example, 99% of all groups of order 2000 are 2-groups

6 Motivation Hello! Why p-groups? Outline Resources Important aspects of p-groups Some comments on p-groups Folklore conjecture: almost all groups are p-groups Sylow Theorem: every nontrivial group has p-groups as subgroups Nilpotent groups: direct products of p-groups Solvable groups: iterated extensions of p-groups Counterpart to theory of finite simple groups Challenge: classify p-groups... Many reductions to p-groups exist: Restricted Burnside Problem, cohomology, Schur multiplier, p-local subgroups,... p-groups are fascinating and accessible to computations! So let s do it...

7 Motivation Hello! Why p-groups? Outline Resources Outline of this lecture series 1 motivation 2 pc presentations Go there 3 p-quotient algorithm Go there 4 p-group generation Go there 5 classification by order Go there 6 isomorphisms Go there 7 automorphisms Go there 8 coclass theory Go there

Eick, E. A. O Brien Chapman & Hall/CRC, 2005 The p-group generation algorithm E. A. O Brien J.

8 Motivation Hello! Why p-groups? Outline Resources thanks to E. A. O Brien for providing some slides Main resources Handbook of computational group theory D. Holt, B. Eick, E. A. O Brien Chapman & Hall/CRC, 2005 The p-group generation algorithm E. A. O Brien J. Symb. Comp. 9, (1990) Heiko Dietrich (heiko.dietrich@monash.edu) Computational aspects of finite p-groups ICTS, Bangalore 2016

9 pc presentations Go to Overview Go to p-quotient Algorithm

10 Groups and computers How to describe groups in a computer? For example, the dihedral group D 8 can be defined as a permutation group... matrix group... finitely presented group G = (1, 2, 3, 4), (1, 3) ; G = ( ), ( ) GL 2(3); G = r, m r 4, m 2, r m = r 3. Best for p-groups: (polycyclic) presentations!

11 Group presentations Let F be the free group on a set X ; let R be a set of words in X X 1. If R = R F is the normal closure of R in F, then G = F/R is the group defined by the presentation {X R} with generators X and relators R; we also write G = X R and call X R a presentation for G. Informally, it is the largest group generated by X and satisfying the relations R. Example 1 relator {}}{ Let X = {r, m} and R = {r 4, m 2, m 1 rmr 3 }, and G = X R = r, m r 4, m 2, } r m {{ = r } 3. relation What can we say about G? Well... r m = r 3 means rm = mr 3, so: G = {m i r j i = 0, 1 and j = 0, 1, 2, 3}, so G 8; D 8 = r, m with r = (1, 2, 3, 4) and m = (1, 3) satisfies R; thus G = D 8.

12 Group presentations Problem: many questions are algorithmically undecidable in general; eg is X R finite, trivial, or abelian? is a word in X trivial in X R? However: group presentations are very compact definitions of groups; many groups from algebraic topology arise in this form; some efficient algorithms exist, eg so-called quotient algorithms ; (see also C. C. Sims: Computation with finitely presented groups, 1994) many classes of groups can be studied via group presentations. Let s discuss how to define p-groups by a useful presention!

13 Background: central series Center If G is a p-group, then its center Z(G) = {g G h G: g h = g} is non-trivial. This leads to the upper central series of a p-group G defined as 1 = ζ 0 (G) < ζ 1 (G) <... < ζ c (G) = G where ζ 0 (G) = 1 and each ζ i+1 (G) is defined by ζ i+1 (G)/ζ i (G) = Z(G/ζ i (G)); it is the fastest ascending series with central sections. Related is the lower central series G = γ 1 (G) > γ 2 (G) >... > γ c+1 (G) = 1 where γ 1 (G) = G and each γ i+1 (G) is defined as 1 γ i+1 (G) = [G, γ i (G)]; it is the fastest descending series with central sections. The number c is the same for both series; the (nilpotency) class of G. 1 As usual, [A, B] = [a, b] a A, b B where [a, b] = a 1 b 1 ab = a 1 b a

14 Example: central series Example 2 Let G = D 16 = r, m with r = (1, 2, 3, 4, 5, 6, 7, 8), m = (1, 3)(4, 8)(5, 7). Then G has class c = 3; its lower central series is G > r 2 > r 4 > 1 and has sections 2 G/γ 2 (G) = C 2 C 2, γ 2 (G)/γ 3 (G) = C 2, and γ 3 (G) = C 2. We can refine this series so that all section are isomorphic to C 2 : G > r > r 2 > r 4 > 1. In general: every central series of a p-group G can be refined to a composition series G = G 1 > G 2 >... > G n+1 = 1 where each G i G and G i /G i+1 = Cp ; thus G is a polycyclic group. 2 If n is a positive integer, then C n denotes a cyclic group of size n.

15 Polycyclic groups Polycyclic group The group G is polycyclic if it admits a polycyclic series, that is, a subgroup chain G = G 1... G n+1 = 1 in which each G i+1 G i and G i /G i+1 is cyclic. Polycyclic groups: solvable groups whose subgroups are finitely generated. Example 3 The group G = (2, 4, 3), (1, 3)(2, 4) = Alt(4) is polycyclic with series G = G 1 > G 2 > G 3 > G 4 = 1 where G 2 = (1, 3)(2, 4), (1, 2)(3, 4) = V 4 G 1 G 3 = (1, 2)(3, 4) G 2 Each G i /G i+1 is cyclic, so there is g i G i \ G i+1 with G i /G i+1 = g i G i+1 ; for example, g 1 = (2, 4, 3), g 2 = (1, 3)(2, 4), g 3 = (1, 2)(3, 4).

16 Polycyclic Sequence Polycyclic sequence Let G = G 1... G n+1 = 1 be a polycyclic series. A related polycyclic sequence X with relative orders R(X) is X = [g 1,..., g n ] with R(X) = [r 1,..., r n ] where each g i G i \ G i+1 and r i = g i G i+1 = G i /G i+1. A polycyclic series is also called pcgs (polycyclic generating set). Important observation: each G i = g i, g i+1,..., g n and G i = r i r n. Example 4 Let G = D 16 = r, m with r = (1, 2, 3, 4, 5, 6, 7, 8) and m = (1, 3)(4, 8)(5, 7). Examples of pcgs: X = [m, r] with R(X) = [2, 8]: G = m, r > r > 1; X = [m, r, r 4 ] with R(X) = [2, 4, 2]: G = m, r, r 4 > r, r 4 > r 4 > 1; X = [m, r, r 3, r 2 ] with R(X) = [2, 1, 2, 4]; note that r, r 3, r 2 = r 3, r 2.

17 Normal Forms Lemma: Normal Form Let X = [g 1,..., g n ] be a pcgs for G with R(X) = [r 1,..., r n ]. If g G, then g = g e1 1 gen n for unique e i {0,..., r i 1}. We call g = g e1 1 gen n the normal form with respect to X. Proof. Let g G be given; we use induction on n. If n = 1, then G = g 1 = C r1 and the lemma holds; now let n 2. Since G/G 2 = g 1 G 2 = C r1, we can write gg 2 = g e1 1 G 2 for a unique e 1 {0,..., r 1 1}, that is, g = g e1 1 g G 2. X = [g 2,..., g n ] is pcgs of G 2 with R(X ) = [r 2,..., r n ], so by induction g = g e1 1 g = g e2 2 gen n for unique e i {0,..., r i 1}. In conclusion, g = g e1 1 gen n as claimed.

18 Example: Normal Forms Example 5 A pcgs of G = Alt(4) with R(X) = [3, 2, 2] is X = [g 1, g 2, g 3 ] where g 1 = (1, 2, 3), g 2 = (1, 2)(3, 4), g 3 = (1, 3)(2, 4). This yields G = G 1 > G 2 > G 3 > G 4 = 1 with each G i = g i,..., g 3. Now consider g = (1, 2, 4) G. First, we have gg 2 = g 2 1G 2, so g = g 2 1 g = (1, 4)(2, 3) G 2. Second, g G 3 = g 2 G 3, so g = g 1 2 g = (1, 3)(2, 4) = g 3 G 3. In conclusion, g = g 2 1g = g 2 1g 2 g = g 2 1g 2 g 3.

19 Polycyclic group to presentation Let G be group with pcgs X = [g 1,..., g n ] and R(X) = [r 1,..., r n ]; define G i = g i,..., g n. There exist a,j, b,,j {0, 1,..., r j 1} with: g ri i g gj i = g ai,i+1 i+1 g ai,n n (for all i, since G i /G i+1 = g i G i+1 = C ri ) = g bi,j,j+1 j+1 g bi,j,n n (for all j < i, since g i G j+1 G j ). A polycyclic presentation (PCP) for G Let H = x 1,..., x n R such R contains exactly the above relations: x ri i = x ai,i+1 i+1 x ai,n n and x xj i = x bi,j,j+1 j+1 x bi,j,n n. Then H = G with pcgs X = [x 1,..., x n ] and R(X) = [r 1,..., r n ]. Proof. Define ϕ: H G by x i g i. The elements g 1,..., g n satisfy the relations in R, so ϕ is an epimorphism by von Dyck s Theorem. By construction, H is polycyclic with pcgs X and order at most G. Thus, ϕ is an isomorphism.

20 Polycyclic group to presentation Example 6 Let G = Alt(4) with pcgs X = [g 1, g 2, g 3 ] and R(X) = [3, 2, 2] where g 1 = (1, 2, 3), g 2 = (1, 2)(3, 4), g 3 = (1, 3)(2, 4). Then g 3 1 = g 2 2 = g 2 3 = 1, g g1 2 = g 2g 3, g g1 3 = g 2, g g2 3 = g 3, and so G = x 1, x 2, x 3 x 3 1 = x 2 2 = x 2 3 = 1, x x1 2 = x 2x 3, x x1 3 = x 2, x x2 3 = x 3. Theorem Every pcgs determines a unique polycyclic presentation; every polycyclic group can be defined by a polycyclic presentation.

21 Pc presentation to group Polycyclic presentation (pcp) A presentation x 1,..., x n R is a polycyclic presentation with power exponents s 1,..., s n N if the only relations in R are x si i = x ai,i+1 i+1 x ai,n n (all i, each a i,k {0,..., s k 1) x xj i = x bi,j,j+1 j+1 x bi,j,n n (all j < i, each b i,j,k {0,..., s k 1). We write Pc x 1,..., x n R and omit trivial commutator relations x xj i = x i. The group defined by a pc-presentation is a pc-group. Theorem If G = Pc x 1..., x n R with power exps [s 1,..., s n ], then X = [x 1,..., x n ] is a pcgs of G. If g G, then g = x e1 1 xen n for some e i {0,..., s i 1}. Careful: (x i G i ) si = 1 only implies that r i = G i /G i+1 divides s i, not r i = s i ; so in general R(X) = [r 1,..., r n ] [s 1,..., s n ].

22 Consistent pc presentations Note: Only power exponents (not relative orders) are visible in pc presentations. Example 7 Let G = Pc x 1, x 2, x 3 x 3 1 = x 3, x 2 2 = x 3, x 5 3 = 1, x x1 2 = x 2x 3 ; this is a pc-group with pcgs X = [x 1, x 2, x 3 ] and power exponents S = [3, 2, 5]. We show R(X) = [3, 2, 1], so G = 6: First, note that x 10 2 = x 5 3 = 1, so x Second, x x1 2 = x 2x 3 = x 3 2 so x 27 2 = x (x3 1 ) 2 = x x3 2 = x(x2 2 ) 2 = x 2, and thus x This implies that 5 x 2, and forces x 3 = 1 in G. Note that x 0 1x 0 2x 0 3 = 1 = x 0 1x 0 2x 1 3 are two normal forms (wrt power exponents). Consistent pc presentation A pc-presentation with power exponents S is consistent if and only if every group element has a unique normal form with respect to S; otherwise it is inconsistent. How to check consistency? use collection and consistency checks!

23 Collection Let G = Pc x 1,..., x n R with power exponents S = [s 1,..., s n ]. Consider a reduced word w = x e1 i 1 x er i r, that is, each i j i j+1 ; we can assume e j N, otherwise eliminate using power relations. Collection Let w = x e1 i 1 x er i r as above and use the previous notation: the word w is collected if w is the normal form wrt S, that is, i 1 <... < i r and each e j {0,..., s ij 1}; if w is not collected, then it has a minimal non-normal subword of w, that is, a subword u of the form u = x ej i j x ij+1 with i j > i j+1, eg u = x 2 3x 1 or u = x si j i j eg u = x 5 2 with s 2 = 5. Collection is a method to obtain collected words.

24 Collection algorithm Let G = Pc x 1,..., x n R with power exponents S = [s 1,..., s n ]. Consider a reduced word w = x e1 i 1 x er i r, that is, each i j i j+1 ; we can assume e j N, otherwise eliminate using power relations. Collection algorithm Input: polycyclic presentation Pc x 1,..., x n R and word w in X Output: a collected word representing w Repeat the following until w has no minimal non-normal subword: choose minimal non-normal subword u = x si j i j or u = x ej i j x ij+1 ; if u = x si j i j, then replace u by a suitable word in x ij+1,..., x n ; if u = x ej i j x ij+1, then replace u by x ij+1 u with u word in x ij+1,..., x n. Theorem The collection algorithm terminates.

25 Collection algorithm If w contains more than one minimal non-normal subword, a rule is used to determine which of the subwords is replaced (making the process well-defined). Collection to the left: move all occurrences of x 1 to the beginning of the word; next, move all occurrences of x 2 left until adjacent to the x 1 s, etc. Collection from the right: the minimal non-normal subword nearest to the end of a word is selected. Collection from the left: the minimal non-normal subword nearest to the beginning of a word is selected.

26 Example: collection Consider the group D 16 = Pc x1, x 2, x 3, x 4 x 2 1 = 1, x 2 2 = x 3 x 4, x 2 3 = x 4, x 2 4 = 1, x x1 2 = x 2x 3, x x1 3 = x 3x 4. Aim: collect the word x 3 x 2 x 1. Since power exponents are all 2, we only use generator indices: to the left from the right from the left 321 = = = 231 = = = 2134 = = = = = = 1244 = = = 12 = 1244 = 1244 = 12 = 12

27 Consistency checks Theorem 8: consistency checks Pc x 1,..., x n R with power exps [s 1,..., s n ] is consistent if and only if the normal forms of the following pairs of words coincide x k (x j x i ) and (x k x j )x i for 1 i < j < k n, (x sj j )x i and x sj 1 j (x j x i ) for 1 i < j n, x j (x si i ) and (x jx i )x si 1 i for 1 i < j n, x j (x sj j ) and (xsj j )x j for 1 j n, where the subwords in brackets are to be collected first. Example 9 If G = Pc x 1, x 2, x 3 x 3 1 = x 3, x 2 2 = x 3, x 5 3 = 1, x x1 2 = x 2x 3, then (x 2 2)x 1 = x 3 x 1 = x 1 x 3 and x 2 (x 2 x 1 ) = x 2 x 1 x 2 x 3 = x 1 x 2 2x 2 3 = x 1 x 3 3. Since x 1 x 3 = x 1 x 3 3 are both normal forms, the presentation is not consistent. Indeed, we deduce that x 3 = 1 in G.

28 Weighted power-commutator presentation So far we have seen that every p-group can be defined via a consistent polycyclic presentation. However, the algorithms we discuss later require a special type of polycyclic presentations, namely, so-called weighted power-commutator presentations.

29 Weighted power-commutator presentation A weighted power-commutator presentation (wpcp) of a d-generator group G of order p n is G = Pc x 1,..., x n R such that {x 1,..., x d } is a minimal generating set G and the relations are x p j = n k=j+1 xα(j,k) k (1 j n, 0 α(j, k) < p) [x j, x i ] = n k=j+1 xβ(i,j,k) k (1 i < j n, 0 β(i, j, k) < p) note that every G i = x i,..., x n is normal in G. Moreover, each x k {x d+1,..., x n } is the right side of some relation; choose one of these as the definition of x k.

30 Weighted power-commutator presentation Example 10 Consider G = Pc x 1,..., x 5 x 2 1 = x 4, x 2 2 = x 3, x 2 3 = x 5, x 2 4 = x 5, x 2 5 = 1 [x 2, x 1 ] = x 3, [x 3, x 1 ] = x 5. Here {x 1, x 2 } is a minimal generating set of G, and we choose: x 3 has definition [x 2, x 1 ] and weight 2; x 4 has definition x 2 1 and weight 2; x 5 has definition [x 3, x 1 ] and weight 3.

31 Weighted power-commutator presentation Why are (w)pcp s useful? consistent pcp s allow us to solve the word problem for the group: given two words, compute their normal forms, and compare them the additional structure of wpcp s allows more efficient algorithms: for example: consistency checks, p-group generation (later) a wpcp exhibits a normal series G > G 1 >... > G n = 1: many algorithms work down this series and use induction: first solve problem for G/G k, and then extend to solve the problem for G/G k+1, and so eventually for G = G/G n.... how to compute wpcp s? p-quotient algorithm (next lecture)

32 Conclusion Lecture 1 Things we have discussed in the first lecture: polycyclic groups, sequences, and series polycyclic generating sets (pcgs) and relative orders polycyclic presentations (pcp), power exponents, and consistency normal forms and collection consistency checks weighted polycyclic presentations (wpcp)

Isomorphism Testing Standard Presentations Example. Isomorphism testing. Go to Classifications. Go to Automorphisms

Isomorphism Testing Standard Presentations Example Isomorphism testing Go to Classifications Go to Automorphisms Isomorphism Testing Standard Presentations Example Conclusion Lecture 3 Things we have discussed