Journal of Sciences, Islamic Republic of Iran 24(4): 361 - 364 (2013) http://jsciences.ut.ac.ir University of Tehran, ISSN 1016-1104 Some properties of -capable and -perfect groups M. R. Rismanchian* Department of Pure Mathematics, Faculty of Mathematical Sciences, Shahrekord University, Shahrekord, Islamic Republic of Iran. Received: 7 July 2013 / Revised: 28 October 2013 / Accepted: 16 November 2013 Abstract In this article we introduce the notion of n-capable groups. It is shown that every group G admits a uniquely determined subgroup (〖Z^n)〗^* (G) which is a characteristic subgroup and lies in the n-centre subgroup of the group G. This is the smallest subgroup of G whose factor group is n-capable. Moreover, some properties of n-central extension will be studied. Keywords: n-central; n-capable; n-perfect; n-unicentral. Introduction (i) G = {1} ⟺Z (G) =G, In 1979 Fay and Waals [3] introduced the notion of the (ii) (G ∕ N) =∕N, n-potent and the n-centre subgroups of a group G, for a (iii) N ⊆ Z(G) ⟺ [N, G] =1, positive integer n, respectivelyas follows: (iv) Z(G×H) =Z(G) ×Z(H). G =< [x, y ]| x, y ∈ G >, Materials and Methods Z(G) = {x∈G| xy =yx, ∀y ∈ G}, . -capability Baer [1] initiated an investigation of the question where [x, y ]=x y xy . It is easy to see that G is a "which conditions a group G must be fulfill in order to fully invariant subgroup and Z(G) is a characteristic be isomorphic with the group of inner automorphisms of subgroup of group G. In the case n=1, these subgroups a group E? As InnE ≅ E⁄ (E), it is equivalent to will be G and Z(G), the drive and centre subgroups of study when G ≅ E/Z(E). By Hall and Senior [4] such a G, respectively. If G =G, then G is said to be n-perfect. group is called capable. Let n be a positive integer, this Let H be a subgroup of G, then [H, G] is defined as notion can be generalized as follows: follows: Definition . A group G is said to be n-capable if there exists a group E such that G ≅ E/Z(E). Consider [H, G] =< [h, g]| h ∈ H, g ∈ G >, the homomorphism ψ ∶E⟶G such that Z(E) includes the kernel of ψ. The intersection of all subgroups of G and in particular if H = G, we get G. The following of the form ψ(Z (E)), for every such ψ, denoted by lemma is similar to the Lemma 2.1 of [5]. ()∗(). Lemma . Let G and H be two groups and N be a The group G is said to be n-unicentral if ()∗() = normal subgroup of G. Then (). It is easy to see that ()∗() is a characteristic * Corresponding author: Tel /Fax: +983814421622; Email: [email protected] 361 Vol. 24 No. 4 Autumn 2013 M. R. Rismanchian J. Sci. I. R. Iran subgroup of included in (), see [6]. The following theorem is useful in the sense that the Therefore [ℎ, ℎ ] =1,∀ℎ ∈ and then ℎ ∈ (). quotient group by ()∗() is -capable which is a Now we have ( ()) ⊆ . Thus by the definition generalized version of the work of Beyl, Feglner and ( )∗() ⊆ Z(E) ⊆ , which completes the Schmid in [2] and similar to the work of Mirebrahimi proof. and Mashayekhy [7] in the case of varieties of groups, see also [8] for more investigations. An immediate necessary and sufficient condition for a group G to be n-capable is, Theorem . Let be normal subgroup of and / be -capable (∈ ). If =⋂∈ , then / Corollary . A group G is n-capable if and only is -capable. if (Z)∗(G) =1. Proof. By definition of -capability, for any ∈, there Now we have a sufficient condition for n-capability of a exists the following short exact sequence group. ⊂ 1→ () → → / → 1. Corollary 1.5. Let N be a normal subgroup of G, such ∗ that N⋂(Z ) (G) =1. If G/N is n-capable, then so is G. Let B=∏∈ (), and The next theorem shows that the class of n-capable A = {(e) ∈ ∏∈ | ∃ g ∈ G s. t Ψ(e) =gH,∀i∈I}, groups is closed under the direct product which generalizes Proposition 6.3 of [2]. A group G is said to Where ∏∈ is the cartesian product of the groups be subdirect product of the groups {G}∈, if G is a , . Clearly ⊆ . For any ∈, we can choose the subgroup of the (unrestricted) direct product , elements e, such that Ψe,=gH. Thus e = ∏∈ G such that p(G) =G,i∈I, where p s are natural projections. e,∈ ∏∈ . Also it is clear that the map G∕N⟶A∕B, Theorem 1.6. Let G be a subdirect product of the n- gN ⟶ e B capable groups {G}∈. Then so is G. Proof. Since is n-capable, we have the following is an isomorphism. Now, as =(), we conclude short exact sequences, that / is -capable. ⊂ ψ Theorem 1.3. ()∗() is the least subgroup lies in the 1→Z (E) →E →G → 1, i ∈ I. -centre of such that /()∗() is an -capable group. Define Proof. Let 1 → → → → 1 be an -central ψ =ψ :E G, ∈ {}↦{ψ()} extension by , i.e. ⊆ (). ∈ ∈ By isomorphism and Theorem 2.2z it is clear that /()∗() is -capable. Now let be a normal and let E = ψ (G),A=∏∈ Z (E). Then A is the n- subgroup of , where / is -capable. Therefore, central subgroup of ∏∈ E. Hence we obtain the there exists an -central extension following commutative diagram, ψ| 1 → () ⟶ → / →1. 1 → A → E → G → 1 ↓ ↓ ↓ ψ Let E = {(g, h) ∈ G × H | gN = φ (h)} and be the 1 → ∏∈ Z (E)→ ∏∈ E → ∏∈ G → 1 , projection map (, ℎ) ⟼ . Then where ψ| is the restricted map of ψ and the vertical maps 1 → → → →1 ∏ ∏ E→ ∈ E and G→ ∈ G are inclusions. Since G is a is -central extension, since ( × ) = () × subdirect product and kerψ ⊆E, the group E is a (). Let (, ℎ) ∈ (), ( ,ℎ )∈× such subdirect product of {E}∈. Now it is obvious that A ⊆ Z(E). For the reveres that (ℎ)=. Thus, we have inclusion, let {e}∈ ∈Z (E) and t ∈E for an arbitrary ′ fixed group E. Denote also p to be the natural (1,1) = [( , ℎ), ( ,ℎ) ]=([ , ], [ℎ, ℎ ]). ′ projection for E.Therefore, there exists {t }∈ ∈E such 362 Some properties of n-capable and n-perfect groups ′ ′ The following important result is immediate. that p {t }∈ =t . Thus p′ {e } , t′ = p′ ([e ,t′] )=p′ ({1 } ) =1. Corollary . For any free presentation 1 → R → ∈ ∈ ∈ ∈ π F →G→1 of G, we have On the other hand, ∗ ∗ p′([{e } , {t′ } ]) =[p′ ({e } ),p′ ({t′ } )] (Z ) (G) =(Z ) (F⁄)[R, F ] . ∈ ∈ ∈ ∈ = [p′ ({e } ),p′ ({t′ } ] ∈ ∈ . − = [e ,t ]. The concept of covering of a central extension by Hence, [e,t ] =1 and so the reverse inclusion holds. another central extension has been studied in page 92 of By A= Z (E), we get the n-capability of G, which [6]. Here we generalize this notion. completes the proof. Let e: 1 → A → H →G→1, be an n-central extension The following corollary is immediate. by the group . Now we state the following definition. () Corollary . If ∏ G is a weak direct product of ∈ Definition . We say that the n-central extension e the groups {G}∈, then () n-central extension, ∗ () () ∗ (Z ) (∏∈ G)⊆∏(∈ Z ) (G). ψ 1→A →H →G⟶1, . If there exists a (unique) homomorphism θ:H→ The structure of Z∗(G) by any free presentation for the H such that the following diagram is commutative, group G is given in [2]. In this section in a similar way, ∅ we study the structure of (Z)∗(G). First, we give the 1→A → H → G →1 following useful lemma. ↓ θ| ↓ θ ↓ I π 1→A →H →G→1. Lemma . Let 1→R→F→G→1 be a free presentation of the group G, and 1→A→B→C→ The n-central extension e is said to be , if 1 be an n-central extension of a group C. If α:G→C is a uniquely covers any other n-central extension by the homomorphism, then there exists a homomorphism group G. β:F∕[R,F]→B such that the following diagram is commutative: The following useful lemma can be easily proved. π lemma . Let G be an n-perfect group. Then 1→ 1 → R ∕ [R, F ]→ F∕[R,F ] → G → 1 1→G→G→1, is a universal n-central extension if and ↓ β| ↓ β ↓ α ψ only if any n-central extension by splits. 1 → A → B → C → 1 Now, we present the following theorem which states Where π is the natural homomorphism induced by π and some essential properties of universal -central β| is the restriction of β. extension. Theorem . For any free presentation 1→R→ π Theorem . Let e :1→A →H →G→1,i=1,2, be F →G→1, and every n-central extension 1 → A → φ n-central extensions by the group G. Then E →G→1, we have (i) If and are universal -central extensions, then ∗ ∗ π(Z ) (F⁄)[R, F ] ⊆φ(Z ) (E). there exists a homomorphism → such that maps onto , φ Proof. By Lemma 2.1 and putting 1→A→E→G→1 (ii) If e is universal n-central extension, then Hand G instead of the second row in the diagram, there exists a are n-perfect, homomorphism β:F⁄[R, F] →E such that the (iii) If 1→1→H→G→1, is a universal -central corresponding diagram is commutative. It is easily to extension, then so is 1 → 1 → G → G → 1.