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1 Cover Page The handle holds various files of this Leiden University dissertation Author: Stanojkovski, M. Title: Intense automorphisms of finite groups Issue Date:

2 Chapter 10 Obelisks Let p > 3 be a prime number. A p-obelisk is a finite p-group G for which the following hold. 1. The group G is not abelian. 2. One has G : G 3 = p 3 and G 3 = G p. The following proposition will immediately clarify our interest in p-obelisks. Proposition 318. Let p > 3 be a prime number and let G be a finite p-group of class at least 4. If int(g) > 1, then G is a p-obelisk. Proof. Combine Theorems 164 and 189. Chapter 10 will be entirely devoted to understanding the structure of p-obelisks and that of their subgroups. Some of the results, especially coming from Section 10.4, are rather technical and their relevance will become evident in Chapter Some properties We remind the reader that, if p is a prime number and G is a finite p-group, then wt G (i) = log p G i : G i+1 where (G i ) i 1 denotes the lower central series of G. Lemma 319. Let p > 3 be a prime number and let G be a p-obelisk. Let (G i ) i 1 denote the lower central series of G. Then the following hold. 1. The class of G is at least One has wt G (1) = 2 and wt G (2) = The group G/G 3 is extraspecial of exponent p. 115

3 Proof. The group G is non-abelian and thus G 2 G 3. The index G : G 3 being equal to p 3, it follows from Lemma 31 that wt G (1) = 2 and wt G (2) = 1. We denote now G = G/G 3 and use the bar notation for the subgroups of G. Then G 2 is contained in Z(G) and G 2 = Z(G), as a consequence of Lemma 27. The exponent of G is p, because G p is contained in G 3. Lemma 320. Let p > 3 and let G be a p-obelisk. Then G is regular. Proof. This follows directly from Lemma 53. Proposition 321. Let p > 3 be a prime number and let G be a p-obelisk. Let (G i ) i 1 be the lower central series of G and let c denote the class of G. Then the following hold. 1. For all i Z 1, one has wt G (i) wt G (i + 1) If wt G (i) wt G (i + 1) = 1, then i = c For all positive integers k and l, not both even, one has [G k, G l ] = G k+l. Proof. Proposition 321 is a simplified version of Theorem 4.3 from [Bla61], which can also be found in Chapter 3 of [Hup67] as Satz We remark that the term p-obelisk does not appear in [Bla61] or [Hup67] and is of our own invention. Moreover, originally Proposition 321(1-2) was phrased in the following way: if G is a p-obelisk, then (wt G (i)) i 1 = (2, 1, 2, 1,..., 2, 1, f, 0, 0,...) where f {0, 1, 2}. Lemma 322. Let p > 3 be a prime number and let G be a p-obelisk. Let c denote the class of G and let i {1,..., c 1}. Then the following hold. 1. The index i is odd if and only if wt G (i) = The index i is even if and only if wt G (i) = If wt G (c) = 2, then c is odd. 4. If c is even, then wt G (c) = 1. Proof. For all j {1,..., c 1}, denote w j = wt G (j). Thanks to Lemma 319, we have w 1 = 2 and w 2 = 1. As a consequence of Proposition 321, whenever i < c 1, the product w i w i+1 is equal to 2 and, for all indices i, j {1,..., c 1}, one has w i = w j if and only if i and j have the same parity. It follows from Proposition 321(1) that wt G (c) can be 2 only if c is odd. We recall that, if G is a p-group, then ρ denotes the map x x p on G. 116

4 Lemma 323. Let p > 3 be a prime number and let G be a p-obelisk. Then, for all i, k Z >0, one has ρ k (G i ) = G 2k+i. In his original proof of Proposition 321, Blackburn also proves Lemma 323. Blackburn s proof strongly relies on the fact that p-obelisks are regular and it makes use of some technical lemmas that can be found in [Hup67, Ch. III]. Proposition 324. Let p > 3 be a prime number and let G be a p-obelisk. Let (G i ) i 1 be the lower central series of G and let c denote its nilpotency class. Then Z(G) = G c. Proof. We work by induction on c. If c = 2, then, by Lemma 319(3), the group G is extraspecial so G 2 = Z(G). Assume now that c > 2. The subgroup G c is central, because G has class c, and, by the induction hypothesis, Z(G/G c ) = G c 1 /G c. It follows that G c Z(G) G c 1 and Z(G) G c 1. Moreover, by Proposition 321(1), the width wt G (c 1) is either 1 or 2. If wt G (c 1) = 1, then Z(G) = G c ; we assume thus that wt G (c 1) = 2. By Lemma 322(1), there exists a positive integer k such that c 1 = 2k + 1 so, from Lemma 323, we get G c 1 = ρ k (G) and G c = ρ k (G 2 ). As a consequence of Proposition 321(1), the subgroup G c has order p. Let us assume by contradiction that Z(G) G c, in other words G c 1 : Z(G) = Z(G) : G c = p. Let N = C G (G c 1 ). The commutator map G/G 2 G c 1 /G c G c is bilinear by Lemma 24 and it factors as a surjective non-degenerate map G/N G c 1 / Z(G) G c. It follows from Lemma 2 that G/N is cyclic of order p so, by Lemma 28, one has G 2 = [N, G]. Lemma 54 yields ρ k ([N, G]) = [N, ρ k (G)] = [N, G c 1 ] = {1} and so G c = ρ k (G 2 ) = {1}. Contradiction. Lemma 325. Let G be a group and let N be a normal subgroup of G. Let moreover H and K be subgroups of G such that K H. Then one has (H N)K = (KN) H. Proof. Easy exercise. Lemma 326. Let p > 3 be a prime number and let G be a p-obelisk. Then each non-abelian quotient of G is a p-obelisk. Proof. Let N be a normal subgroup of G such that G/N is not abelian. We claim that N is contained in G 3. Denote first H = G/N. Then we have H : H 2 G : G 2. Moreover, H being non-abelian, Lemma 31 yields H : H 2 p 2, and therefore, from Lemma 319(2), it follows that N G 2. If N G 3 = N, then N is contained in G 3 and we are done. Assume by contradiction that N G 3 N. As a consequence of Lemma 319(2), the subgroup N does not contain G 3. Let now 117

5 M be a normal subgroup of G such that N G 3 M G 3 and G 3 : M = p, as given by Lemma 35. Then G = G/M has class 3 and N {1}. But, by Lemma 140, the centre of G is equal to G 3 so, G 3 having order p, Lemma 29 yields G 3 N. In particular, G 3 is contained in MN. Thanks to Lemma 325, we get that G 3 = (G 3 N)M = M, which gives a contradiction. It follows that N G 3, as claimed, and thus we have H : H 3 = G : G 3. It is moreover clear that H p = H 3, and so we have proven that H is a p-obelisk. Lemma 327. Let p > 3 be a prime number and let G be a p-obelisk. Then the following hold. 1. If H is a quotient of G of class i, then Z(H) = H i. 2. Let N be a subgroup of G. Then N is normal in G if and only if there exists i Z >0 such that G i+1 N G i. Proof. (1) Let H be a quotient of G and let i denote the class of H. Let moreover c denote the class of G and note that i c. If i = 0 or i = 1, the group H is abelian and Z(H) = H. Assume now that i > 1. Then H is a non-abelian quotient of a p-obelisk so, by Lemma 326, it is a p-obelisk itself. To conclude, apply Proposition 324. For the proof of (2), we combine (1) with Lemma Power maps and commutators Throughout Section 10.2 we will faithfully follow the notation from the List of Symbols. In particular, if p is a prime number and G is a finite p-group, then ρ denotes the map G G that is defined by x x p. We remind the reader that ρ is in general not a homomorphism. Lemma 328. Let p > 3 be a prime number and let G be a p-obelisk. Then the following hold. 1. For all i, k Z >0 the map ρ k : G i G i induces a surjective homomorphism ρ k i : G i/g i+1 G 2k+i /G 2k+i For all h, k Z >0 not both even, the commutator map induces a bilinear map γ h,k : G h /G h+1 G k /G k+1 G h+k /G h+k+1 whose image generates G h+k /G h+k+1. Proof. (1) Let i and k be positive integers and, without loss of generality, assume that G 2k+i+1 = {1}. We work by induction on k and we start by taking k = 1. As a consequence of Lemma 20, the group [G i, G i ] is contained in G 2i so, from Lemma 323, it follows that [G i, G i ] p is contained in G 2i+2. The index i being positive, G 2i+2 is contained in G i+3 = {1}. Now, the prime p is larger than 3 so G ip is also 118

6 contained in G i+3 = {1}. It follows from Lemma 20, that (G i ) p is contained in G ip, and so, thanks to Corollary 48, the map ρ : G i G i is a homomorphism. The function ρ factors as a surjective homomorphism ρ 1 i : G i/g i+1 G i+2, thanks to Lemma 323. This finishes the proof for k = 1. Assume now that k > 1 and define ρ k i = ρ 1 2k+i 1 ρ 1 2k+i 3... ρ 1 i+2 ρ 1 i. As a consequence of the base case, the map ρ k i is a surjective homomorphism ρ k i : G i/g i+1 G 2k+i /G 2k+i+1 and, by its definition, it is induced by ρ k. This proves (1). To prove (2) combine Proposition 321(3) with Lemma 23. Corollary 329. Let p > 3 be a prime number and let G be a p-obelisk. Let c be the class of G. Let moreover i and j be integers of the same parity such that 1 i j c and one of the following holds. 1. The number j is even. 2. One has wt G (j) = 2. Define m = j i 2. Then the map ρm : G i G i induces an isomorphism of groups ρ m i : G i /G i+1 G j /G j+1. Proof. By Lemma 328(1), the map ρ m : G i G i induces a surjective homomorphism ρ m i : G i /G i+1 G j /G j+1. Now, i and j having the same parity, it follows from Lemma 322 that wt G (i) = wt G (j) and ρ m i is a bijection. Lemma 330. Let p > 3 be a prime number and let G be a p-obelisk. Denote by c the class of G. Let moreover h and k be positive integers, not both even, such that h + k c. Assume additionally that, if h + k is odd, then wt G (h + k) = 2. Then the map γ h,k from Lemma 328 is non-degenerate. Proof. Without loss of generality, assume that c = h + k and so G h+k+1 = {1}. We prove non-degeneracy of γ h,k by looking at the parity of h + k. Assume first that h + k is odd and, without loss of generality, h is odd and k is even. From Lemma 322, it follows that wt G (h) = 2 and wt G (k) = 1. Moreover, by assumption, wt G (h + k) = 2. Since the image of γ h,k generates G h+k, the map γ h,k is nondegenerate. Let now h + k be even. The numbers h and k are both odd so wt G (h) = wt G (k) = 2, by Lemma 322(2). Assume without loss of generality that h k. Then, by Lemma 323, the set ρ k h 2 (G h ) coincides with the subgroup G k. Let now C = C Gh (G k ) and D = C Gk (G h ). Since γ h,k 1, Lemma 20 yields that G h+1 C G h and G k+1 D G k. The commutator map induces a nondegenerate map G h /C G k /D G h+k so, wt G (h+k) being equal to 1, Lemma 2 yields that G h : C = G k : D. Now, by Lemma 320, the group G is regular, and therefore so is C. Thanks to Lemma 52(1), the set ρ k h 2 (C) is a subgroup of C and 119

7 so, thanks to Lemma 54, one has [ρ k h 2 (C), G h ] = [C, ρ k h 2 (G h )] = [C, G k ] = {1}. In particular, ρ k h 2 (C) D. Since Gh : C = G k : D and wt G (h) = wt G (k) = 2, we derive from Corollary 329 that ρ k h 2 (C) = D. Assume now by contradiction that there exists x G h such that G h = x, C. Then G k = ρ k h 2 (x), D and therefore, the commutator map being alternating, one has G h+k = [G k, G h ] = [x, ρ k h 2 (x)] = {1}. Contradiction to the class of G being h + k. It follows that the quotient G h /C is not cyclic and so C = G h+1 and D = G k+1. In particular, γ h,k is non-degenerate. Corollary 331. Let p > 3 be a prime number and let G be a p-obelisk. Denote by c the class of G. Let moreover l {1,..., c 1} be such that c l is odd. Then the map G c l /G c l+1 Hom(G l /G l+1, G c ) that is defined by is a surjective homomorphism of groups. t G c l+1 (x G l+1 [t, x]) Proof. As a consequence of Lemma 323, the groups G l /G l+1, G c l /G c l+1, and G c are elementary abelian and the map γ c l,l from Lemma 328 is thus a bilinear map of F p -vector spaces. Respecting the notation from Section 1.1, we define δ : G c l /G c l+1 Hom(G l /G l+1, G c ) to be the map sending each element v G c l /G c l+1 to v (γ c l,l ). In other words, if v = t G c l+1, then δ(v) : G l /G l+1 G c is defined by x G l+1 [t, x]. As a consequence of Lemma 328(2), the function δ is a homomorphism of groups and δ differs from the zero map. Let us now, for all i {1,..., c}, denote w i = wt G (i). It follows that the dimension of Hom(G l /G l+1, G c ) is equal to w l w c and, if w l w c = 1, then δ is surjective. We assume that w l w c 1. The index c l being odd, it follows that either l or c is even. Proposition 321 yields w c l = w l w c and, if l is even, then w c = 2. As a consequence of Lemma 330, the map δ is injective and so δ is also surjective Framed obelisks Let p > 3 be a prime number and let G be a p-obelisk. Then G is framed if, for each maximal subgroup M of G, one has Φ(M) = G 3. Lemma 332. Let p > 3 be a prime number and let G be a p-obelisk. Let moreover h, k Z >0, with h odd and k even, and n Z 0. Then the following diagram is commutative. 120

8 G h /G h+1 G k /G k+1 γ h,k Gh+k /G h+k+1 (id Gh /G h+1, ρ n k ) G h /G h+1 G k+2n /G k+2n+1 ρ n h+k γ h,k+2n Gh+k+2n /G h+k+2n+1 Proof. The maps from the above diagram are defined in Lemma 328. Assume without loss of generality that G h+k+2n+1 = {1} so that G h+k+2n is central. The diagram is clearly commutative for n = 0. We will prove the most delicate case, i.e. when n = 1, and leave the general case to the reader. Set n = 1. Let (x, y) G h G k. We will show, and that suffices, that [x, y p ] = [x, y] p. Thanks to Lemma 18(4), one gets [x, y] p [x, y p ] = [[y r, x], y]. Applying Lemma 20 twice, one gets that, for each index r, the element [[y r, x], y] belongs to G h+2k, which is itself contained in the central subgroup G h+k+2. Moreover, the group G h+k being central modulo G h+k+1, Lemma 22 yields [y r, x] [y, x] r mod G h+k+1. Thanks to Lemma 23, the commutator map on G induces a bilinear map G h+k /G h+k+1 G k /G k+1 G h+2k and therefore we get [[y r, x], y] = [[y, x] r, y] = [[y, x], y] r. It follows that r=1 [x, y] p [x, y p ] = [[y r, x], y] = [[y, x], y] r = [[y, x], y] 2) (p r=1 and, the prime p being larger than 2, the number ( p 2) is a multiple of p. Since [[y, x], y] belongs to G h+k+2, it follows from Lemma 323 that [x, y p ] = [x, y] p. This concludes the case n = 1. Lemma 333. Let p > 3 be a prime number and let G be a p-obelisk. Let moreover h, k Z >0, with h odd and k even, and m Z 0. Then the following diagram is commutative. r=1 G h /G h+1 G k /G k+1 γ h,k Gh+k /G h+k+1 (ρ m h, id G k /G k+1 ) G h+2m /G h+2m+1 G k /G k+1 ρ m h+k γ h+2m,k Gh+k+2m /G h+k+2m+1 121

9 Proof. The maps in the diagram are as in Lemma 328 and they are well-defined. Assume without loss of generality that G h+k+2m+1 = {1} and so G h+k+2m is central. Let (x, y) G h G k. The diagram is clearly commutative if m = 0; we will prove commutativity when m = 1, the most difficult case, and we will leave the general case to the reader. Set m = 1. We will prove, and that suffices, that [x p, y] = [x, y] p. Applying Lemma 18(3) twice, we get [x p, y][x, y] p = [x, [x p s, y]] [ x, ( p s 1 j=1 [x, [x p s j, y]] ) [x, y] p s]. Thanks to Lemma 20, each element [x, [x p s j, y]] belongs to G k+2h and, the group G h+k+3 being trivial, G k+2h centralizes G h+k. Again by Lemma 20, for each index s, the element [x, y] p s belongs to G h+k and applying Lemma 18(1) twice yields [x p, y][x, y] p = = [ p s 1 x, j=1 [ p s 1 x, j=1 [x, [x p s j, y]] [x, [x p s j, y]] ] [x, [x, y] p s ] ] [x, [x, y]] p s. Thanks to Lemma 23, given any two positive integers i and j, the commutator map induces a bilinear map G i /G i+1 G j /G j+1 G i+j /G i+j+1. By taking consecutively (i, j) = (h, k) and (i, j) = (h, h + k), we get respectively that [x p s j, y] [x, y] p s j mod G h+k+1 and so [x, [x p s j, y]] [x, [x, y] p s j ] [x, [x, y]] p s j mod G 2h+k+1. By taking (i, j) = (h, 2h + k), we derive that [x p, y][x, y] p = = [ p s 1 x, j=1 [x, [x, y]] p s j] [x, [x, y]] p s [x, [x, [x, y]] (p s 2 ) ] [x, [x, y]] p s = [x, [x, [x, y]]] (p s [x, [x, y]] p s = [x, [x, [x, y]]] 3) (p [x, [x, y]] ( p 2). 2 ) The prime p being larger than 3, both ( p 2) and ( p 3) are multiples of p. As both [x, [x, y]] and [x, [x, [x, y]]] belong to G h+k+1, it follows from Lemma 323 that [x p, y] = [x, y] p. This concludes the proof for m =

10 Proposition 334. Let p > 3 be a prime number and let G be a p-obelisk. Let moreover h, k Z >0, with h odd and k even, and let m, n Z 0. Then the following diagram is commutative. G h /G h+1 G k /G k+1 γ h,k Gh+k /G h+k+1 (ρ m h, ρn k ) G h+2m /G h+2m+1 G k+2n /G k+2n+1 ρ m+n h+k γ h+2m,k+2n Gh+k+2(m+n) /G h+k+2(m+n)+1 Proof. Combine Lemmas 332 and Lemma 333. Lemma 335. Let p > 3 be a prime number and let G be a p-obelisk of class at least 3. Let moreover M be a maximal subgroup of G. Then [M, M] = [M, G 2 ] and, whenever wt G (3) = 2, the following are equivalent. 1. One has Φ(M) G One has [M, M] = M p = Φ(M). Proof. The subgroups M p and [M, M] are both characteristic in the normal subgroup M; thus both M p and [M, M] are normal in G. By Lemma 319(2), the quotient G/G 2 has order p 2 and so G : M = M : G 2 = p. It follows from Lemma 28 that [M, M] = [M, G 2 ] and so, as a consequence of Corollary 329 and Lemma 330, the least jumps of [M, M] and M p in G are both equal to 3 and of width 1. In particular, Φ(M) is contained in G 3 and Lemma 327(2) yields G 4 M p [M, M]. If the third width of G is equal to 2, then it follows that Φ(M) G 3 if and only if [M, M] = Φ(M) = M p. We remark that, as a consequence of Lemma 323, quotients of consecutive elements of the lower central series of a p-obelisk are vector spaces over F p and therefore, in (2) and (3) from Proposition 336, it makes sense, for each positive integer i, to talk about subspaces of G i /G i+1. Proposition 336. Let p > 3 be a prime number and let G be a p-obelisk. Then the following conditions are equivalent. 1. The p-obelisk G is framed. 2. For each 1-dimensional subspace l of G/G 2, the quotient G 3 /G 4 is generated by ρ 1 1(l) and γ 1,2 ({l} G 2 /G 3 ). 3. For each h, k Z >0, with h odd and k even, and for each 1-dimensional subspace l in G h /G h+1, the spaces ρ k/2 h (l) and γ h,k({l} G k /G k+1 ) generate G h+k /G h+k

11 Proof. (1) (2) Let π : G G/G 2 denote the natural projection. Then, through π, there is a bijection between the maximal subgroups of G and the 1-dimensional subspaces of G/G 2. For any maximal subgroup M of G, we know from Lemma 335 that [M, G 2 ] = [M, M] and therefore (2) holds if and only if, given any maximal subgroup M of G, one has Φ(M)G 4 = G 3. Lemma 327(2) yields that (2) is satisfied if and only if, for any maximal subgroup M of G, one has Φ(M) = G 3. We now deal with (2) (3). The implication is proven by taking h = 1 and k = 2, so we will prove that (2) implies (3). Let l be a 1-dimensional subspace of G h /G h+1. Define moreover m = h 1 2, n = k 2 h+k 3 2, and S = m + n = 2. Thanks to Lemma 328(1), there exists a 1-dimensional subspace l of G/G 2 such that ρ m 1 (l ) = l and, moreover, ρ n 2 (G 2/G 3 ) = G k /G k+1. By assumption G 3 /G 4 is generated by ρ 1 1 (l ) and γ 1,2 ({l } G 2 /G 3 ), so it follows from Lemma 328(1) that ρ S 3 (ρ 1 1(l )) and ρ S 3 (γ 1,2 ({l } G 2 /G 3 )) together span G h+k /G h+k+1. We now have ρ S 3 (ρ 1 1(l )) = ρ S+1 1 (l ) = ρ k/2 (l) and, thanks to Proposition 334, we also have ρ S 3 (γ 1,2({l } G 2 /G 3 )) = γ h,k (ρ m 1 (l ) ρ n 2 (G 2/G 3 )) = γ h,k ({l} G k /G k+1 ). This completes the proof. h 10.4 Subgroups of obelisks The major goal of this section is to link structural properties of subgroups of a p-obelisk to the parities and widths of their jumps. The importance of Section 10.4 will become clear in Chapter 13. Proposition 337. Let p > 3 be a prime number and let G be a p-obelisk. Let H be a subgroup of G that is itself a p-obelisk. Then H = G. Proof. The subgroup H is non-abelian, by definition of a p-obelisk, and it is in particular non-trivial. Let l denote the least jump of H in G. Then, as a consequence of Lemma 20, the subgroup H 2 = [H, H] is contained in G 2l. Moreover, since H p is equal to H 3, the subgroup H p is contained in H 2. It follows from Corollary 329 that the minimum jump of H 2 is at most l+2: we get that 2l l+2 and therefore l 2. We will show that HG 2 = G. Assume by contradiction that G HG 2. Then, as a consequence of Lemma 319(2), the width wt G H (l) is equal to 1 and so Lemma 28 yields that H 2 = [H, H G l+1 ]. Thanks to Lemma 20, the subgroup H 2 is contained in G 2l+1 and therefore 2l + 1 l + 2. It follows that l = 1 and that H 2 is contained in G 3. Define now G = G/G 4 and use the bar notation for the subgroups of G. By the isomorphism theorems, the groups H and H/(H G 4 ) are isomorphic and so, as a consequence of Lemma 326, the group H is abelian or 124

12 a p-obelisk. The minimum jump of H p in G being equal to 3, we have that 3 is a jump of H 2 in G and so H is a p-obelisk. Now, the group G 3 is central in G and so, the group H 2 being non-trivial, the quotient H/(H G 3 ) is not cyclic. It follows that 2 is a jump of H in G and, from the combination of Lemmas 322 and 328(2), that H 2 has order p. Since H 2 contains H p, we get H 2 = H p = H 3. Contradiction to H being non-abelian. We have proven that G = HG 2, from which we derive G = HΦ(G). Lemma 33 yields H = G. Lemma 338. Let p > 3 be a prime number and let G be a p-obelisk. Let H be a cyclic subgroup of G. Then all jumps of H in G have the same parity and width 1. Proof. Let H be a cyclic subgroup of G. Then, for all i Z >0, there exists k Z 0 such that H G i = H pk. Moreover, i Z >0 is a jump of H in G if and only if there exists k {0, 1,..., log p H 1} such that H G i = H pk and H G i+1 = H pk+1. We conclude thanks to Lemma 328(1). Lemma 339. Let p > 3 be a prime number and let G be a p-obelisk. Let c denote the nilpotency class of G and assume that one of the following holds. 1. The number c is even. 2. One has wt G (c) = 2. If H is a subgroup such that all of its jumps in G have the same parity and width 1, then H is cyclic. Proof. Without loss of generality we assume that H is non-trivial and we take l to be the least jump of H in G. Let moreover J (H) denote the collection of jumps of H in G and define J = {l + 2k : k Z 0, k (c l)/2}. Let x be an element of H such that dpt G (x) = l; the existence of x is guaranteed by Lemma 82. Write K = x and let J (K) be the collection of jumps of K in G. By assumption J contains J (H) and, as a consequence of Corollary 329, the set J is contained in J (K). Keeping in mind that each jump of H in G has width 1, one derives K = j J (K) p wtgk(j) j J p wtgk(j) j J It follows that K = H and H is cyclic. p wtg H (j) j J (H) p wtg H (j) = H. Lemma 340. Let p > 3 be a prime number and let G be a p-obelisk. Let c denote the nilpotency class of G and let H be a subgroup of G such that H G c = {1}. If all jumps of H in G have the same parity and width 1, then H is cyclic. 125

13 Proof. We denote G = G/G c and we will use the bar notation for the subgroups of G. As a consequence of Lemma 428, the group G is abelian or it is a p-obelisk. If G is abelian, then c = 2 and so, by Lemma 339, the subgroup H is cyclic. Assume now that G is non-abelian and thus a p-obelisk. The group G has class c 1 and, as a consequence of Corollary 322, either c 1 is even or wt G (c 1) = 2. It follows from Lemma 339 that H is cyclic and, the intersection H G c being trivial, so is H. Lemma 341. Let p > 3 be a prime number and let G be a p-obelisk. Let c denote the nilpotency class of G and let H be a non-trivial subgroup of G such that H G c = {1}. Let l be the least jump of H in G and assume that all jumps of H in G have the same parity and the same width. Then the following hold. 1. The group H is abelian. 2. One has Φ(H) = H G l+1. Proof. Let J (H) denote the collection of jumps of H in G. We first assume wt G H (l) = 1. By Lemma 340, the subgroup H is cyclic and Φ(H) has index p in H. It follows that Φ(H) = H G l+1. Assume now that wt G H (l) = 2. Then, thanks to Lemma 322(3), the jump l is odd. The subgroup [H, H] is contained in G 2l, thanks to Lemma 20, and therefore, 2l being even, Lemma 328(2) yields 2l > c. In particular, one has [H, H] = {1} so Φ(H) = H p. Moreover, as a consequence of Lemma 328(1), the set of jumps of H p in G is equal to J (H) \ {l} and each jump of H p has width 2. It follows that H p = H G l+1. Thanks to Proposition 321 the width wt G H (l) is either 1 or 2 and the proof is thus complete. Lemma 342. Let p > 3 be a prime number and let G be a p-obelisk. Let c be the class of G and let H be a non-trivial subgroup of G such that H G c = {1}. Denote by l the least jump of H and assume that H G l+1 = Φ(H). Finally, assume that c l is odd. Then, for each complement K of G c in HG c, there exists t G c l such that K = tht 1. Proof. The subgroup G c is central in G, because G has class c, and so, by Lemma 114, all complements of G c in T = HG c are of the form {f(h)h : h H} as f varies in Hom(H, G c ). The subgroup G c is elementary abelian, as a consequence of Lemma 323, and therefore Hom(H, G c ) is naturally isomorphic to Hom(H/Φ(H), G c ) = Hom(H/(H G l+1 ), G c ). By assumption, c l is odd so, thanks to Corollary 331, the homomorphism G c l /G c l+1 Hom(G l /G l+1, G c ), defined by tg c l (xg l+1 [t, x]), is surjective. By Lemma 323, the quotient G l /G l+1 is elementary abelian and therefore the restriction map 126 Hom(G l /G l+1, G c ) Hom(HG l+1 /G l+1, G c )

14 is surjective. By the isomorphism theorems, HG l+1 /G l+1 and H/(H G l+1 ) are isomorphic and so every homomorphism H G c is of the form x [t, x], for some t G c l. For each complement K of G c in T there exists thus t G c l such that K = {[t, x]x : x H} = {txt 1 : x H} = tht

15 128

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