# ON GENERALIZED RELATIVE COMMUTATIVITY DEGREE OF A FINITE GROUP. A. K. Das and R. K. Nath

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1 International Electronic Journal of Algebra Volume 7 (2010) ON GENERALIZED RELATIVE COMMUTATIVITY DEGREE OF A FINITE GROUP A. K. Das and R. K. Nath Received: 12 October 2009; Revised: 15 December 2009 Communicated by A. Çiğdem Özcan Abstract. Given any two subgroups H and K of a finite group G and an element g G the aim of this article is to study the probability that the commutator of an arbitrarily chosen pair of elements (one from H and the other from K) equals g. Mathematics Subject Classification (2000): 20C15 20D60 20P05 Keywords: commutativity degree commutators group characters 1. Introduction Throughout this paper G denotes a finite group H and K two subgroups of G and g an element of G. In [1] Erfanian et al. have considered the probability Pr(H G) for an element of H to commute with an element of G. On the other hand in [8] Pournaki et al. have studied the probability Pr g (G) that the commutator of an arbitrarily chosen pair of group elements equals g (a generalization of this notion can be found also in [7]). The main object of this paper is to further generalize these notions and study the probability that the commutator of a randomly chosen pair of elements (one from H and the other from K) equals g. In other words we study the ratio Pr g (H K) = {(x y) H K : xyx 1 y 1 = g} (1) and further extend some of the results obtained [1] and [8]. In the final section with H normal in G we also develop and study a character theoretic formula for Pr g (H G) which generalizes the formula for Pr g (G) given in ([8] Theorem 2.1). In the process we generalize a classical result of Frobenius (see [2]). Note that if H = K = G then Pr g (H K) = Pr g (G) which coincides with the usual commutativity degree Pr(G) of G if we take g = 1 the identity element of G. It may be recalled (see for example [3]) that Pr(G) = k(g) G where k(g) denotes the number of conjugacy classes of G. On the other hand if K = G and g = 1 then Pr g (H K) = Pr(H G).

2 GENERALIZED RELATIVE COMMUTATIVITY DEGREE Some basic properties and a computing formula Let [H K] denote the subgroup of G generated by the commutators [x y] = xyx 1 y 1 with x H and y K. Also for brevity let us write Pr 1 (H K) = Pr(H K). Clearly Pr(H K) = 1 [H K] = {1} and Pr g (H K) = 0 g / {[x y] : x H y K}. It is also easy to see that if C K (x) = {1} for all x H {1} then Pr(H K) = 1 H + 1 K 1. (2) The following proposition says that Pr g (H K) is not very far from being symmetric with respect to H and K. Proposition 2.1. Pr g (H K) = Pr g 1(K H). However if g 2 = 1 or if g H K (for example when H or K is normal in G) we have Pr g (H K) = Pr g (K H) = Pr g 1(H K). Proof. The first part follows from the fact that [x y] 1 = [y x]. On the other hand for the second part it is enough to note that if g H then (x y) (y 1 yxy 1 ) and if g K then (x y) (xyx 1 x 1 ) define bijective maps between the sets {(x y) H K : [x y] = g} and {(y x) K H : [y x] = g}. Pr g (H K) respects the Cartesian product in the following sense. Proposition 2.2. Let G 1 and G 2 be two finite groups with subgroups H 1 K 1 G 1 and H 2 K 2 G 2. Let g 1 G 1 and g 2 G 2. Then Pr (g1 g 2 )(H 1 H 2 K 1 K 2 ) = Pr g1 (H 1 K 1 )Pr g2 (H 2 K 2 ). Proof. It is enough to note that for all x 1 y 1 G 1 and for all x 2 y 2 G 2 we have [(x 1 x 2 ) (y 1 y 2 )] = ([x 1 y 1 ] [x 2 y 2 ]). We now derive a computing formula which plays a key role in the study of Pr g (H K). Theorem 2.3. Pr g (H K) = 1 g 1 x Cl K (x) C K (x) = 1 H g 1 x Cl K (x) where Cl K (x) = {yxy 1 : y K} the K-conjugacy class of x. 1 Cl K (x)

3 142 A. K. DAS AND R. K. NATH Proof. We have {(x y) H K : xyx 1 y 1 = g} = {y K : [x y] = g}. Note that for any x H we have T x φ g 1 x Cl K (x). {x} T x where T x = Let T x φ for some x H. Fix an element y 0 T x. Then y gy 1 0 y defines a one to one correspondence between the set T x and the coset gc K (x). This means that T x = C K (x). Thus we have {(x y) H K : xyx 1 y 1 = g} = T x = The first equality in the theorem now follows from (1). g 1 x Cl K (x) C K (x). For the second equality consider the action of K on G by conjugation. Then for all x G we have This completes the proof. Cl K (x) = orb(x) = K : stab(x) = K C K (x). (3) As an immediate consequence we have the following generalization of the wellknown formula Pr(G) = k(g) G. Corollary 2.4. If H is normal in G then Pr(H K) = k K(H) H where k K (H) is the number of K-conjugacy classes that constitute H. Proof. Note that K acts on H by conjugation. The orbit of any element x H under this action is given by Cl K (x) and so H is the disjoint union of these classes. Hence we have Pr(H K) = 1 1 H Cl K (x) = k K(H) ; H noting that for g = 1 the condition g 1 x Cl K (x) is superfluous. The Schur-Zassenhaus Theorem (see [6 page 125]) says that if H is a normal subgroup of G such that gcd( H G : H ) = 1 then H has a complement in G. In particular if H is a normal subgroup of G with C G (x) H for all x H {1} then using Sylow s theorems and the fact that nontrivial p-groups have nontrivial centers we have gcd( H G : H ) = 1. So by the Schur-Zassenhaus Theorem H has a complement in G. Such groups belong to a well-known class of groups called the Frobenius Groups; for example the alternating group A 4 the dihedral groups

4 GENERALIZED RELATIVE COMMUTATIVITY DEGREE 143 of order 2n with n odd the nonabelian groups of order pq where p and q are primes with q (p 1). Proposition 2.5. If H is an abelian normal subgroup of G with a complement K in G then Pr g (H G) = Pr g (H K). Proof. Let x H. Since H is abelian we have C HK (x) = {hk : hkx = xhk} = {hk : kx = xk} = HC K (x). Thus C HK (x) = H C K (x). Also since H is abelian and normal Cl K (x) = Cl HK (x). Hence from Theorem 2.3 it follows that 1 Pr g (H G) = H 2 C HK (x) = Pr g (H K). K This completes the proof. g 1 x Cl HK (x) Corollary 2.6. If H is a normal subgroup of G with C G (x) = H for all x H {1} then Pr g (H G) = Pr g (H K) where K is a complement of H in G. In particular Pr(H G) = 1 H + H 1. G Proof. The first part follows from the discussion preceding the above proposition and the second part follows from (2). 3. Some bounds and inequalities We begin with the inequality Pr(H K) C H(K) which follows from (1) using the fact that + C K(H) ( H C H (K) ) (C H (K) K) (H C K (H)) {(x y) H K : xyx 1 y 1 = 1}. On the other hand we have Proposition 3.1. If g 1 then (i) Pr g (H K) 0 = Pr g (H K) C H(K) C K (H)

5 144 A. K. DAS AND R. K. NATH (ii) Pr g (H G) 0 = Pr g (H G) 2 H Z(G) C G(H) H G (iii) Pr g (G) 0 = Pr g (G) 3 G : Z(G) 2. Proof. Let g = [x y] for some (x y) H K. Since g 1 we have x / C H (K) and y / C K (H). Consider the left coset T (xy) = (x y)(c H (K) C K (H)) of C H (K) C K (H) in H K. Clearly T (xy) = C H (K) C K (H) and [a b] = g for all (a b) T (xy). This proves part (i). Similarly part (ii) follows considering the two disjoint cosets T (xy) and T (xyx) with K = G while part (iii) follows considering the three disjoint cosets T (xy) T (xyx) and T (xyx) with H = K = G. As a generalization of Proposition 5.1 of [8] we have Proposition 3.2. with equality if and only if g = 1. Proof. By Theorem 2.3 we have Pr g (H K) = Pr g (H K) Pr(H K) 1 g 1 x Cl K (x) C K (x) 1 C K (x) = Pr(H K). Clearly the equality holds if and only if g 1 x Cl K (x) for all x H that is if and only if g = 1. The following is an improvement to Proposition 5.2 of [8]. Proposition 3.3. Let p be the smallest prime dividing G and g 1. Then Pr g (H K) H C H(K) p H < 1 p. Proof. Without any loss we may assume that C H (K) H. Let x H be such that g 1 x Cl K (x). Then since g 1 we have x / C H (K) and Cl K (x) > 1. But Cl K (x) is a divisor of K and hence of G. Therefore Cl K (x) p. Hence by Theorem 2.3 we have Pr g (H K) 1 H This completes the proof. g 1 x Cl K (x) 1 p H C H(K) < 1 p H p.

6 GENERALIZED RELATIVE COMMUTATIVITY DEGREE 145 Pr(H K) is monotonic in the following sense. Proposition 3.4. If K 1 K 2 are subgroups of G then Pr(H K 1 ) Pr(H K 2 ) with equality if and only if Cl K1 (x) = Cl K2 (x) for all x H. Proof. Clearly Cl K1 (x) Cl K2 (x) for all x H. So by Theorem 2.3 we have Pr(H K 1 ) = 1 1 H Cl K1 (x) 1 1 H Cl K2 (x) = Pr(H K 2). The condition for equality is obvious. Since Pr(H K) = Pr(K H) it follows from the above proposition that if H 1 H 2 and K 1 K 2 are subgroups of G then Pr(H 1 K 1 ) Pr(H 2 K 2 ). Proposition 3.5. If K 1 K 2 are subgroups of G then Pr(H K 2 ) 1 K 2 : K 1 ( Pr(H K 1 ) + K 2 K 1 H K 1 with equality if and only if C H (x) = {1} for all x K 2 K 1. Proof. By Theorem 2.3 We have ( ) 1 Pr(H K 2 ) = C H (x) + C H (x) H K 2 x K 1 x K 2 K 1 ( 1 Pr(H K 1 ) + K ) 2 K 1. K 2 : K 1 H K 1 Clearly the equality holds if and only if x K 2 K 1 ( C H (x) 1) = 0 ) that is if and only if C H (x) = {1} for all x K 2 K 1. In general Pr g (H K) is not monotonic. For example if G = S 3 g = (123) H =< (12) > K 1 =< (1) > K 2 =< (13) > and K 3 = S 3 then Pr g (H K 1 ) = 0 Pr g (H K) = 1 4 Pr g(h K) = 1 6. However we have

7 146 A. K. DAS AND R. K. NATH Proposition 3.6. If H 1 H 2 and K 1 K 2 are subgroups of G then Pr g (H 1 K 1 ) H 2 : H 1 K 2 : K 1 Pr g (H 2 K 2 ) with equality if and only if g 1 x / Cl K2 (x) for all x H 2 H 1 g 1 x / Cl K2 (x) Cl K1 (x) for all x H 1 and C K1 (x) = C K2 (x) for all x H 1 with g 1 x Cl K1 (x). In particular for g = 1 the condition for equality reduces to H 1 = H 2 and K 1 = K 2. Proof. By Theorem 2.3 we have H 1 K 1 Pr g (H 1 K 1 ) = C K1 (x) 1 g 1 x Cl K1 (x) 2 g 1 x Cl K2 (x) C K2 (x) = H 2 K 2 Pr g (H 2 K 2 ). The condition for equality follows immediately. Using Propostion 3.2 we have Corollary 3.7. Pr g (H G) G : H Pr(G) with equality if and only if g = 1 and H = G. The following theorem generalizes Theorem 3.5 of [1]. Theorem 3.8. Let p be the smallest prime dividing G. Then Pr(H K) C H(K) H and Pr(H K) (p 1) C H(K) + H p H + p( H X H C H (K) ) + X H X H ( K p) p where X H = {x H : C K (x) = 1}. Moreover in each of these bounds H and K can be interchanged. Proof. If [H K] = 1 then there is nothing to prove as in that case C H (K) = H and X H equals H or an empty set according as K is trivial or nontrivial.

8 GENERALIZED RELATIVE COMMUTATIVITY DEGREE 147 On the other hand if [H K] 1 then X H C H (K) = φ and so C K (x) = C K (x) + C K (x) + x X H x C H (K) = X H + K C H (K) + C K (x). (X H C H (K)) (X H C H (K)) C K (x) But for all x H (X H C H (K)) we have {1} = C K (x) K which means that p C K (x) K p. Hence using Theorem 2.3 we get the required bounds for Pr(H K). The final statement of the theorem follows from the fact that Pr(H K) = Pr(K H). As a consequence we have Corollary 3.9. Let [H K] {1}. If p is the smallest prime divisor of G then In particular Pr(H K) 3 4. Pr(H K) 2p 1 p 2. Proof. Since [H K] {1} we have K {1} and C H (K) H. So K p and C H (K) H p. Therefore by Theorem 3.8 we have since p 2. Pr(H K) (p 1) C H(K) + H p H p 1 p + 1 = 2p 1 p p One can see that the above bound is best possible. For example consider two non-commuting elements a and b of order p in a nonabelian group G with p as the smallest prime dividing G. Then using (2) for H =< a > and K =< b > we have Pr(H K) = 2p 1 p 2. Proposition Let Pr(H K) = 2p 1 p for some prime p. Then p divides G. 2 If p happens to be the smallest prime divisor of G then In particular H C H (K) = Z p K = and hence H K. C K (H) H C H (K) = Z 2 = K C K (H) if Pr(H K) = 3 4. Proof. The first part follows from the definition of Pr(H K). For the second part by Theorem 3.8 we have 2p 1 p 2 = H : C H (K) p. (p 1) C H(K) + H p H

9 148 A. K. DAS AND R. K. NATH Since p is the smallest prime divisor of G it follows that H : C H (K) = p whence H C H (K) = Z p ; noting that C H (K) H because Pr(H K) = 2p 1 p 2 K have H C K (H) = Z p. Since Z(H) The third part follows from the first two parts. 4. A character theoretic formula 1. Similarly we is never cyclic unless trivial we have H K. In this section all results are under the assumption that H is normal in G. Let ζ(g) denote the number of solutions (x y) H G of the equation [x y] = g. Thus by (1) Pr g (H G) = ζ(g) H G. (4) Our quest for a character theoretic formula for Pr g (H G) starts with the following lemma. Lemma 4.1. ζ(g) defines a class function on G. Proof. It is enough to note that for each a G the map (x y) (axa 1 aya 1 ) defines a one to one correspondence between the sets {(x y) H G : [x y] = g} and {(x y) H G : [x y] = aga 1 }. Thus we have ζ(g) = χ Irr(G) ζ χ χ(g) (5) where Irr(G) denotes the set of all irreducible complex characters of G and denotes the inner product. We now prove the main result of this section. Theorem 4.2. ζ(g) = χ Irr(G) where χ H denotes the restriction of χ to H. H χ H χ(g) Proof. Let χ Irr(G) and let Φ χ be a representation of G which affords χ. Then as in the proof of Theorem 1 of [9] we have by Schur s lemma (see [4] Lemma 2.25 ) Φ χ (yx 1 y 1 ) = G χ(x 1 ) I χ y G where x H and I χ is the identity matrix of size. Multiplying both sides by Φ χ (x) and summing over all x H we get Φ χ (xyx 1 y 1 ) = G Φ χ (x)χ(x 1 ). (xy) H G

10 GENERALIZED RELATIVE COMMUTATIVITY DEGREE 149 Taking trace we have (xy) H G χ(xyx 1 y 1 ) = G χ(x)χ(x 1 ) = χ(g)ζ(g) = G χ H (x)χ H (x) g G = χ ζ = H χ H. Hence in view of (5) the theorem follows. In particular we have Corollary 4.3. ζ is a character of G Proof. It is enough to show that divides H χ H for every χ Irr(G). With notations same as in the proof of the above theorem consider the algebra homomorphism ω χ : Z(C[G]) C given by Φ(z) = ω χ (z) I χ for all z Z(C[G]). Since H is normal in G there exist x 1 x 2... x r H such that H = Cl G(x i ). 1 i r Let K i = Σ x the class sum corresponding to Cl G(x i ) 1 i r. By ([4 x Cl G (x i) Theorem 3.7] and the preceding discussion) ω χ (K i ) is an algebraic integer with Therefore it follows that Thus ω χ (K i ) = χ(x i) Cl G (x i ) 1 i r. H χ H = χ(x)χ(x 1 ) = Cl G (x i ) χ(x i )χ(x 1 1 i r = ω χ (K i )χ(x 1 i ). 1 i r H χ H = ω χ (K i )χ(x 1 i ) 1 i r which is an algebraic integer and hence an integer. This completes the proof. In view of (4) the following character theoretic formula for Pr g (H G) can be easily derived from Theorem 4.2. Pr g (H G) = 1 G χ Irr(G) This formula enables us to strengthen Corollary 3.7 as follows. i ) χ H χ(g). (6)

11 150 A. K. DAS AND R. K. NATH Proposition 4.4. If g G the commutator subgroup of G then Pr g(h G) 1 ( G G : H Pr(G) 1 ) G. Proof. For all χ Irr(G) with = 1 we have χ H = 1 and G ker χ. Also G : G equals the number of linear characters of G. Therefore by (6) Pr g (H G) = 1 G + 1 χ G H χ(g). χ Irr(G) 1 Since χ(g) for all χ Irr(G) we have using Lemma 2.29 of [4] Pr g(h G) 1 G χ H This completes the proof. In particular we have χ Irr(G) 1 1 G ( Irr(G) G : G ) G : H ( = G : H Pr(G) 1 ) G. Corollary 4.5. If G contains a non-commutator (an element which is not a commutator) then Pr(G) 2 G. Proof. The corollary follows by choosing a non-commutator g G and putting H = G. As a consequence we have the following result which is closely related to the subject matter of [5]. Proposition 4.6. If G < p where p is the smallest prime divisor of G then every element of G is a commutator. Proof. It is well-known that Pr(G) 1 G [ ] 1 + G 1 p 2 which in fact can be derived from the inequality G = 2 G : G + p 2 [k(g) G : G ]. χ Irr(G) Hence if G contains a non-commutator it follows using Corollary 4.5 that G p

12 GENERALIZED RELATIVE COMMUTATIVITY DEGREE 151 References [1] A. Erfanian R. Rezaei and P. Lescot On the Relative Commutativity Degree of a Subgroup of a Finite Group Comm. Algebra 35 (12) (2007) [2] F. G. Frobenius Über Gruppencharaktere Gesammelte Abhandlungen Bände III p (J. P. Serre ed.) Springer-Verlag Berlin [3] W. H. Gustafson What is the probability that two group elements commute? Amer. Math. Monthly 80 (1973) [4] I. M. Isaacs Character Theory of Finite Groups Dover Publications Inc. New YorK [5] L. Kappe and R. F. Morse On commutators in groups (English summary) Groups St. Andrews Vol London Math. Soc. Lecture Note Ser. 340 Cambridge Univ. Press Cambridge [6] H. Kurzweil and B. Stellmacher The Theory of Finite Groups An Introduction Universitext Springer New York-Berlin-Heidelberg [7] R. K. Nath and A. K. Das On generalized commutativity degree of a finite group to appear in Rocky Mountain J. Math. [8] M. R. Pournaki and R. Sobhani Probability that the commutator of two group elements is equal to a given element J. Pure and Applied Algebra 212 (2008) [9] T. Tambour The number of solutions of some equations in finite groups and a new proof of Itô s theorem Comm. Algebra 28 (11) (2000) A. K. Das and R. K. Nath Department of Mathematics North-Eastern Hill University Permanent Campus Shillong Meghalaya India. s: (A. K. Das) (R. K. Nath)

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