# FACTORING CERTAIN INFINITE ABELIAN GROUPS BY DISTORTED CYCLIC SUBSETS

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1 International Electronic Journal of Algebra Volume 6 (2009) FACTORING CERTAIN INFINITE ABELIAN GROUPS BY DISTORTED CYCLIC SUBSETS Sándor Szabó Received: 11 November 2008; Revised: 13 March 2009 Communicated by Abdullah Harmancı Abstract We will prove that two results on factoring finite abelian groups into a product of subsets, related to Hajós s and Rédei s theorems, can be extended for certain infinite torsion abelian groups Mathematics Subject Classification (2000): Primary 20K01; Secondary 05B45, 52C22, 68R05 Keywords: factorization of finite abelian groups, periodic factorization, fullrank factorization, Hajós-Rédei theory 1 Introduction Let G be a finite abelian group and let A 1,, A n be subsets of G If the product A 1 A n is direct and is equal to G, then we say that the equation G = A 1 A n is a factorization of G A subset A of G in the form A = {e, a, a 2,, a r 1 } is called a cyclic subset of G In order to avoid trivial cases we assume that r 2 and that a r Clearly A is a subgroup of G if and only if a r = e It is a famous result of G Hajós [3] that if a finite abelian group is factored as a direct product of its cyclic subsets, then at least one of the factors must be a subgroup A subset A of G in the form A = {e, a, a 2,, a i 1, a i d, a i+1,, a r 1 } is called a distorted cyclic subset Here we assume that a i d a j for each j, 0 j r 1 If d = e then clearly a distorted cyclic subset coincides with a cyclic subset A D Sands [5] showed that if a finite abelian group is a direct product of distorted cyclic subsets, then at least one of the factors must be a subgroup This is a generalization of Hajós s theorem We will show that Sands s result holds for certain infinite groups too A subset A of G is called normalized if e A If each A i is normalized then we say that the factorization G = A 1 A n is normalized The next theorem of L

2 96 SÁNDOR SZABÓ Rédei is one of the most striking results of the factorization theory of abelian group If a finite abelian group is factored into normalized subsets of prime cardinalities, then at least one the factors must be a subgroup We say that a subset A of G is periodic if there is an element g G \ {e} such that Ag = A The element g is called a period of A Rédei s theorem can be reformulated in terms of periodic subsets in the following way If G = A 1 A n is a factorization of the finite abelian group G and each A i is a prime, then at least one of the factors must be periodic Examples show that the condition that each A i is a prime cannot be dropped from the theorem However, for 2-groups K Amin, K Corrádi and A D Sands ([1] Theorem 15) proved a slightly more general version Namely, if G = BA 1 A n is a factorization of the finite abelian 2-group G such that B = 4 and A 1 = = A n = 2, then at least one of the factors is periodic S Szabó [7] extend the above result proving that if G = BA 1 A n is a factorization of the finite abelian group G such that B = 4 and each A i is a prime, then at least one of the factors must be periodic In this paper we will show that the result holds for a class of infinite torsion abelian groups We define factorizations for infinite groups We assume that each factor in a factorization contains the identity element In short we consider only normalized factorizations of an infinite group Let A i, i I be a collection of finite subsets of an abelian group G such that e A i for each i, i I If each element g G can be written in the form g = a i, a i A i i I uniquely, where only finitely many of the a i s are not equal to the identity element e, then we say that G is factored into its subsets A i, i I We also will say that the equation G = A i i I is a factorization of G Let p be a prime The multiplicative group of all (p α )th roots of unity will be denoted by C(p ) The group C(p ) is the so-called Prüfer group We would like to point out a difference between factoring finite and infinite groups For a finite abelian group G if the product of its subsets A and B is direct and if A B is equal to G, then the product AB is a factorization of G, that is, each element g G is uniquely represented in the form g = ab, a A, b B This does not hold for infinite groups To see why let G = AB be a normalized factorization of the infinite abelian group G, where A is an infinite subset of G Set A = A \ {a}, where a A \ {e} Plainly, A B is equal to G As the product

3 FACTORING CERTAIN INFINITE ABELIAN GROUPS 97 AB is direct it follows that the product A B is also direct On the other hand, the product A B cannot be equal to G However, for finite A this phenomenon cannot occur E J Eigen and V S Prasad [2] have proved the following Let G = AB be a factorization of an abelian group, where A is finite If A is a subset of G such that A = A and the product A B is direct, then G = A B is a factorization of G We will use a corollary of this result Note that the product AB is direct if and only if A 1 A BB 1 = {e} Therefore if the product AB is direct then so is the product A 1 B Thus if A is finite, then A can be replaced by A 1 in each factorization G = AB to get the factorization G = A 1 B Here A 1 = {a 1 : a A} and in general A t = {a t : a A} for each integer t 2 Distorted cyclic factors A D Sands [5] proved that in a factorization of a finite abelian group a distorted cyclic factor always can be replaced by an associated cyclic subset A weaker version of this result holds for infinite abelian groups too Lemma 21 Let G be an abelian group Let A = {e, a, a 2,, a i 1, a i d, a i+1,, a r 1 } be a distorted cyclic subset of G, where C = {e, a, a 2,, a r 1 } is a cyclic subset associated with A If r 4, then in the normalized factorization G = AB the factor A can be replaced by C to get the normalized factorization G = CB Proof We distinguish the following two cases Case 1: i = r 1 Case 2: 1 i r 2 Let us settle case 1 first As G = AB is a factorization of G the sets B, ab,, a r 2 B, a r 1 db (1) form a partition of G Multiplying the factorization G = AB by a we get the factorization G = ag = (aa)b of G The sets ab,, a r 2 B, a r 1 B, a r db (2) form a partition of G Comparing the two partitions we get B a r 1 db = a r 1 B a r db

4 98 SÁNDOR SZABÓ If a r 1 db a r db, then B ab which contradicts (1) Thus a r 1 db a r db = and so db B In the factorization G = AB replace the finite factor A by A 1 to get the factorization G = A 1 B One can draw the conclusion that d 1 B B Multiplying by d we get B db From db B and B db it follows that B = db Plugging this to (1) we get that the sets B, ab,, a r 2 B, a r 1 B form a partition of G and so G = CB is a factorization of G Let us turn to case 2 We would like to prove that G = AB is a factorization of G, that is, the sets B, ab, a 2 B,, a r 1 B (3) form a partition of G Since G = AB is a factorization of G the sets B, ab,, a i 1 B, a i db, a i+1 B,, a r 1 B (4) form a partition of G In particular a u C a v C = for each u, v, u v, 0 u, v r 1 whenever u i and v i In order to verify that (3) is a partition of G we establish that B = db We do this by showing that db B and B db The containment db B follows from the fact that a u B a i B = holds for each u, u i, 0 u r 1 To prove this assume on the contrary that a u C a i C for some u, u i, 0 u r 1 We distinguish two cases depending on u < i or i < u Let us consider the u < i case first Multiplying a u B a i B by a k gives that a u+k B a i+k B (5) If 0 u + k, i + k r 1, (6) u + k i, i + k i, (7) then (5) violates partition (4) Plainly (6) is equivalent to u k r 1 i and so there are r 1 i + u + 1 choices for k Using i r 2, 0 u we get 2 = r 1 (r 2) + (0) + 1 r 1 i + u + 1 and so there are at least two choices for k If i r 2 or 0 u, then there are at least three choices for k and we can get a contradiction Thus we may assume that i = r 2 and 0 = u Now with the k = 1 choice (6) and (7) are satisfied and we get a contradiction Finally consider the i < u case Multiplying a i B a u B by a k gives that a i+k B a u+k B (8)

5 FACTORING CERTAIN INFINITE ABELIAN GROUPS 99 If 0 i + k, u + k r 1, (9) i + k i, u + k i, (10) then (8) contradicts to partition (4) Clearly (9) is equivalent to i k r 1 u and so there are r 1 u + i + 1 choices for k Using u r 1, 1 i we get 2 = r 1 (r 1) + (1) + 1 r 1 u + i + 1 and so there are at least two choices for k If u r 1 or 1 i, then there are at least three choices for k and we can get a contradiction Thus we may assume that u = r 1 and 1 = i Now with the k = 1 choice (9) and (10) are satisfied and we get a contradiction Therefore db B as we claimed In the factorization G = AB replace the finite factor A by A 1 to get the factorization G = A 1 B From this factorization we can conclude that d 1 B B or equivalently B db This completes the proof Lemma 22 Let G be an abelian group Let A = {e, a, a 2,, a i 1, a i d, a i+1,, a r 1 } be a distorted cyclic subset of G, where r 4 A is a subgroup of G if and only if d = e and a r = e Proof If d = e, then A is a cyclic subset of G If a r = e, then this cyclic subset is a subgroup of G Next assume that A is a subgroup of G and try to show that d = e and a r = e We distinguish the following three cases Case 1: i = 1 Case 2: i = r 1 Case 3: 2 i r 2 Let us settle case 1 first Now A = {e, ad, a 2,, a r 1 } As a r 2, a r 1 A, it follows that a A If a = ad, then e = d Therefore A is a cyclic subgroup of G and so a r = e, as required Thus we may assume a ad and consequently a {e, a 2,, a r 1 } If a = e, then a 2 = = a r 1 = e which is a contradiction If a = a 2, then a = e This reduces the problem to the earlier case If a = a j, 3 j r 1, then we get the e = a j 1 contradiction

6 100 SÁNDOR SZABÓ Let us turn to case 2 Now A = {e, a, a 2,, a r 2, a r 1 d} As a A, it follows that a r 1 A If a r 1 = a r 1 d, then e = d and so A is a cyclic subgroup of G It follows that a r = e, as required If a r 1 = e, then the cyclic subset {e, a, a 2,, a r 2 } is a subgroup of G Consequently it is a subgroup of A It follows the contradiction that r 1 divides r If a r 1 = a j, 1 j r 2, then we get the a r 2 = a j 1 contradiction We may turn to case 3 Now a A implies a i A If a i = a i d, then e = d and A is a cyclic subgroup of G It follows that a r = e, as required We may assume that a i = a j, where i j We distinguish two cases depending on j < i or i < j Assume first that j < i Multiply a j = a i by a u to get a j u = a i u If 0 j u, i u i 1, (11) then we get a contradiction (11) is equivalent to j u, u 1 There are ( 1) ( j) + 1 = j choices for u Therefore in the j 1 case we are done So we may assume that j = 0 Multiply a 0 = a i by a u to get a u = a i+u If 0 u i 1, i + 1 i + u r 1, then we get a contradiction Note that the u = 1 choice is suitable, as 2 i r 2 Next assume that i < j Multiply a i = a j by a u to get a i+u = a j+u If i + 1 i + u, j + u r 1, (12) then we get a contradiction (12) is equivalent to 1 u, u r 1 j There are (r 1 j) ( 1) + 1 = r j + 1 choices for u If r j + 1 1, then we get a contradiction Equivalently if j r 2, then we get a contradiction Thus we may assume that j = r 1 Multiplying a i = a r 1 by a u we get a i u = a r 1 u If 0 i u i 1, i + 1 r 1 u r 1, then we get a contradiction If i r 3, then the u = 1 choice is suitable Thus we may assume that i = r 2 Now a r 2 = a r 1 This provides the e = a contradiction

7 FACTORING CERTAIN INFINITE ABELIAN GROUPS 101 The reader may notice that in the proof above in case 2 it is enough to assume that r 3 In this particular case a stronger result holds We will need this result and for easier reference we spell it out as a lemma Lemma 23 Let G be an abelian group Let A = {e, a, a 2,, a r 2, a r 1 d} be a distorted cyclic subset of G, where r 3 A is a subgroup of G if and only if d = e and a r = e The distorted cyclic subset A of an abelian group is called a reducible subset if the following hold (1) A is not a subgroup (2) There is a subset B of G such that G = AB is a factorization of G (3) There are non-subgroup distorted cyclic subsets A 1, A 2 for which G = A 1 A 2 B is a factorization of G Lemma 24 Let G be an abelian group and let A be a non-subgroup distorted cyclic subset of G If A = st, s 3, t 3, then A is reducible Proof Suppose there is a subset B of G such that G = AB is a factorization of G Let A = {e, a, a 2,, a i 1, a i d, a i+1,, a r 1 } and let C = {e, a, a 2,, a r 1 } be a cyclic subset associated with A, where r = st Set C 1 = {e, a, a 2,, a s 1 }, C 2 = {e, a s, a 2s,, a (t 1)s }, A 1 = C 1, A 2 = {e, a s, a 2s,, a (t 1)s d} Note that the product C 1 C 2 is direct and is equal to C By Lemma 21, in the factorization G = AB the factor A can be replaced by C to get the factorization G = CB We can read off from the proof that B = db also holds The sets B, ab, a 2 B,, a r 1 B

8 102 SÁNDOR SZABÓ form a partition of G Using C = C 1 C 2 we get that the sets B, a s B, a 2s B,, a (t 2)s B, a (t 1)s B, ab, a 1+s B, a 1+2s B,, a 1+(t 2)s B, a 1+(t 1)s B, a s 1 B, a s 1+s B, a s 1+2s B,, a s 1+(t 2)s B, a s 1+(t 1)s B form a partition of G Using B = db we get that the sets B, a s B, a 2s B,, a (t 2)s B, a (t 1)s db, ab, a 1+s B, a 1+2s B,, a 1+(t 2)s B, a 1+(t 1)s db, a s 1 B, a s 1+s B, a s 1+2s B,, a s 1+(t 2)s B, a s 1+(t 1)s db form a partition of G and therefore G = A 1 A 2 B is a factorization of G If A 1 is a subgroup of G, then a s = e This violates the factorization G = CB Thus A 1 cannot be a subgroup of G If A 2 is a subgroup of G, then, by Lemma 23, a st = a r = e and d = e Consequently, by Lemma 22, A is a subgroup of G This is not the case Thus A 2 is not a subgroup of G This completes the proof Theorem 25 Let p 1,, p s be distinct odd primes and let [ s ] G = H C(p i ), i=1 where H is a finite abelian group of odd order and p i does not divide H for each i, 1 i s If G = A i (13) i=1 is a factorization of G and each A i is a finite distorted cyclic subset of G, then A i is a subgroup of G for some i, 1 i < Proof We claim that q divides H p 1 p s for each prime divisor q of A k for each factor A k of the factorization (13) To prove the claim assume on the contrary that there is a factor A k of the factorization (13) and a prime divisor q of A k such that q does not divide H p 1 p s We may assume that k = 1 since this is only a matter of indexing the factors Setting B = i=2 the factorization (13) can be written in the form G = A 1 B We distinguish the next two cases A i

9 FACTORING CERTAIN INFINITE ABELIAN GROUPS 103 Case 1: A 1 = q Case 2: A 1 = q In case 1 let and let A 1 = {e, a, a 2,, a i 1, a i d, a i+1,, a r 1 } C = {e, a, a 2,, a r 1 } be a cyclic subset associated with A 1 Now plainly A 1 4 and so, by Lemma 21, in the factorization G = A 1 B, the factor A 1 can be replaced by C to get the factorization G = CB There are cyclic subsets of prime cardinality C 1,, C u such that the product C 1 C s is direct and is equal to C Some of C 1,, C u is equal to q We may assume that C 1 = q since this is only a matter of rearranging the factors Because of the structure of G, each element of G has finite order Let m be the order of c Clearly each prime divisor of m divides H p 1 p s Therefore q and m are relatively prime By the Chinese remainder theorem, there is an integer t such that Plainly t is relatively prime to q t 0 (mod m), t 1 (mod q) By Proposition 3 of [4], in the factorization G = C 1 C u B the factor C 1 can be replaced by C t 1 to get the factorization G = C t 1C 2 C u B Note that (c i ) t = (c t ) i = e and so the element e appears in C t 1 with multiplicity q This is a contradiction In case 2, let m be the least common multiple of the orders of the elements of A 1 Define the integer t in the same way as in case 1 Replace A 1 by A t 1 in the factorization G = A 1 B to get G = A t 1B Now we get the contradiction that the element e appears in A t 1 with multiplicity q By the assumption of the theorem H p 1 p s is odd The claim we have just verified gives in particular that each A i is odd In order to prove the theorem assume on the contrary that in factorization (13) none of the factors is a subgroup of G By Lemma 24, in the factorization (13) each A i can be replaced by a product of non-subgroup distorted cyclic subsets whose cardinalities are primes (It may happen that A i is a prime In this case of course we do not replace A i ) We end up with a factorization in which each factors is normalized, has prime cardinality, and is not a subgroup of G For this factorization Theorem 1 of [6] is applicable and implies that one of the factors is a subgroup of G This contradiction completes the proof

10 104 SÁNDOR SZABÓ 3 Factors of prime cardinality In this section we consider factorizations in which each factor with one possible exception has prime cardinality and one factor may have order four Theorem 31 Let [ r ] G = H C(p i ), i=1 where H is a finite abelian group, p 1,, p r are distinct primes, p i does not divide H for each i, 1 i r If G = B A i (14) is a normalized factorization of G such that B = 4 and each A i is a prime, then one of the factors B, A 1, A 2, is periodic Proof Let H be the product of the (not necessarily distinct) primes p r+1,, p s By the assumptions of the theorem i=1 {p 1,, p r } {p r+1,, p s } = For a factor A j of the factorization (14) with A j = p, where p is a prime let A j be the set of the p-components of the elements of A j We claim that in the factorization (14) A j can be replaced by A j In order to prove the claim let m be a common multiple of the orders of the p -components of the elements of A j and let n be a common multiple of the orders of the p-components of the elements A j Such m, n do exist since each element of G has a finite order As m and n are relatively primes by the Chinese remainder theorem, the system of congruences t 0 (mod m), t 1 (mod n), is solvable By Proposition 3 of [4], A j can be replaced by A t j As At j = A j, the claim is proved The fact that A j can be replaced by A j implies that the elements of A j are distinct Further we can conclude that if A j = p, where p is a prime, then p must be one of the primes p 1,, p s It follows in the same manner that the 2- components of the elements of B are distinct These elements form a set B and B = 4 From the factorization G = B we draw further conclusions Let p be one of the primes p 1,, p r If p is odd, then the product of all the A j s with A j = p forms a factorization of the subgroup i=1 A i

11 FACTORING CERTAIN INFINITE ABELIAN GROUPS 105 C(p ) of G If p = 2, then the product of B and all the A j s with A j = p form a factorization of the subgroup C(p ) of G If H is odd, then the product of all the A j s with A j H forms a factorization of H If H is even, then the product of B and all the A j with A j H forms a factorization of H The subgroups of C(p i ) form a chain For each integer j 0 there is a unique subgroup of order p j Let H i,0, H i,1, be all the subgroups of C(p i ) We assume that H i,j = p j i There are factors A i,1, A i,2, among A 1, A 2, such that A i,1 = H i,1, A i,1a i,2 = H i,2, Note that each nonidentity element of A i,j must have order pj i Now let us go back to factorization (14) To prove the theorem we assume the contrary that none of the factors B, A 1, A 2, is periodic Choose a factor A j and assume that A j = p, where p is a prime (We know that p is one of the primes p 1,, p s ) The p-components of the elements of A j are distinct and form a set A j with A j = p If A j is not a subgroup of G, then replace A j by C j = A j If A j is a subgroup of G, then there is an element in A j whose q-component is not the identity element since A j is not a subgroup of G Here q is a prime p q Now A j can be replaced by C j such that C j is not a subgroup of G and the orders of the elements of C j divide pq In other words C j is constructed from the subgroup A j by multiplying some elements of A j by some elements of order q Let us consider the factorization G = B C i Here none of the factors B, C 1, C 2, is periodic For each i, 1 i r there is an integer α(i) such that the orders of the p i -components of the elements of B are less than or equal to p α(i) i and α(i) 1 The elements of C(p i ) whose order is less than or equal to p α(i) i form the unique subgroup H i,α(i) of C(p i ) Set i=1 K = HH 1,α(1) H r,α(r) Clearly, B K Let D 1,, D n be all the C i factors for which C i K We claim that K = BD 1 D n is a factorization of K As B, D 1,, D n K, it is enough to verify that B D 1 D n = K In order to verify this equation let D 1,, D m be the factors among D 1,, D n whose cardinality is one of p r+1,, p s Assume first that 4 H Note that B D m+1 D n = H Further D 1 D m = ( A 1,1 A 1,α(1) ) ( A r,1 A r,α(r) ) = p α(1) 1 p α(r) r = H 1,α(1) H r,α(r)

12 106 SÁNDOR SZABÓ Assume next that 4 H 1,α(1) H r,α(r) Note that D m+1 D n = H and B D 1 D m = H 1,α(1) H r,α(r) Thus K = BD 1 D n is a factorization of the finite abelian group K By Theorem 1 of [7], one of the factors is periodic This contradiction completes the proof References [1] K Amin, K Corrádi, A D Sands, The Hajós property for 2-groups, Acta Math Hungar 89 (2000), [2] S J Eigen and V S Prasad, Solution to a problem of Sands on the factorization of groups, Indag Mathem, 14 (2003), [3] G Hajós, Über einfache und mehrfache Bedeckung des n-dimensionalen Raumes mit einem Würfelgitter, Math Zeit, 47 (1942), [4] A D Sands, Replacement of factors by subgroups in the factorization of abelian groups, Bull London Math Soc, 32 (2000), [5] A D Sands, A note on distorted cyclic subsets, appears in Mathematica Pannonica [6] S Szabó, Factoring an infinite abelian group by subsets, Periodica Math Hungar, 40 (2000), [7] S Szabó, Rédei s theorem with a factor of order four, appears in Hokkaido Math Journal Sándor Szabó Institute of Mathematics and Informatics University of Pécs Ifjúság u Pécs, Hungary

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