A Classification of the Subgroups of the Rationals Under Addition

A Classification of the Subgroups of the Rationals Under Addition Patrick Miller May 16, 2011 1 Abstract In this paper, we examine one of the most fundamental and interesting algebraic structures, the rational numbers, from the perspective of group theory. We delve into subgroups of the rationals under addition, with the ultimate goal of completely classifying their isomorphisms, gaining insight into general torsion- free groups along the way. We will develop an intuitive and efficient approach to describing a subgroup, namely the height function, and use this description to completely classify subgroup isomorphism. Some time will be dedicated to methods of combining subgroups into new subgroups, one each corresponding to the four main algebraic operations. The reader will come to understand the re- lationship between the resulting groups of such operations and the groups used to form them. Finally, we will focus attention on an elegant result that gives a way of identifying an arbitrary torsion-free group as a subgroup of the rationals. 1 2 Introduction There are many insights that stem from a detailed study of the rational numbers from the viewpoint of group theory. First and foremost, the rational numbers are a construction that is central to modern mathematics. Indeed, students learn of their basic nature very early in education. To improve our understand- ing of the way rational numbers function is an important study in algebra. Secondly, the rational numbers are a quintessential torsion-free group, mean- ing a group whose only element of finite order is the identity. Studying the rational numbers can provide important revelations about torsion-free groups as a whole. Furthermore, and of particular interest to this paper, examining the group structure of the rationals can give us specific criteria to determine when an arbitrary torsion-free group is actually isomorphic to a subgroup of the rationals. The earliest roots of these studies arise from the development of group theory, especially through the Nineteenth Century. However, the fundamentals of this branch of mathematics were never thoroughly applied to the rational numbers in order to answer the question \when are two subgroups of the rationals isomorphic" until the Twentieth Century. In 1937, the German mathematician Reinhold Baer published a 54 page paper that accurately answered this question. The paper also contained many results pertaining to other torsion-free groups. Further work on the subject continued throughout the 1950s, 60s, and 70s, though much of the publishing done on the matter was simply alternate descriptions of Baer's conclusions. Finally, in the early 2000s, the mathemati- cians Friedrich L. Kluempen and Denise Reboli published a paper titled \When Are Two Subgroups of the Rational Numbers Isomorphic". The purpose of this publication was to broaden the accessibility of the subject so as to allow for undergraduate students to study it. While the approach taken in Kluempen and 2 Reboli's paper differs from this paper's methods, the fundamentals are the same. Indeed, they are equivalent classifications. While the paper does an excellent job of presenting the material in an understandable fashion, there is obviously a certain level of expected knowledge in order to fully understand the subject. Accordingly, there are many important definitions that will be used throughout this paper that are essential for the reader to understand intimately. First and foremost, the basics of group theory are taken to be understood by the reader. Any undergraduate who has taken an introductory course to abstract algebra or group theory will have the sufficient tools to grasp the group theory concepts presented in this paper. Secondly, there is a great deal of number theory involved in this topic, and readers should be familiar with basic ideas from this branch of mathematics as well. Unless otherwise noted, the definitions used in this paper will be those presented in Charles C. Pinter's second edition of "A Book of Abstract Algebra". We will now introduce certain definitions that are specific to the study of subgroups of the rational numbers under addition. Firstly, any use of the word \subgroups" in this paper will be referring to subgroups of the rationals under addition. Secondly, if a subgroup contains the unity (1), we will call this group unitary. Note that this definition should not be confused with the definition of a unitary ring; we are referring, here, to the multiplicative identity, although m the group's operation is addition. The multiplication of a rational number n m m m by an integer z will be defined as + + ::; + . Usually, but not always, n n n | {z } z times subgroups will be assigned to the variables A or B, functions will be assigned to the variables h or k, and integers or natural numbers will be given the names m; n; p; or q. 3 3 Subgroup and Homomorphism Structure In this section, we will examine some of the general structural nature of subgroups of the rational numbers. Many of the questions we will address are very typical of group theory. For example, what do homomorphisms and isomorphisms between two subgroups look like? These studies will prove very valuable in studying subgroup isomorphism. The first step towards our goal is to briefly examine the nature of elemen- tal inclusion in subgroups of the rationals. What are some testable conditions under which a rational number is an element of a specific group? It turns out that this question is fairly easily answered. Before we begin, however, it must be stated that the subgroups in the proofs within this section will always be assumed to be unitary. m 1 Theorem 1.1: For relatively prime m; n, n 2 A if and only if n 2 A. m Proof: First, assume that n 2 A. Since m and n are relatively prime, there 1 xm m are x; y 2 Z such that 1 = xm + yn ! n = n + y. n is in A. Addition- ally, x; y 2 A since A contains 1, and, inductively, all of the integers. Thus, xm 1 n + y = n 2 A. 1 1 1 Now, assume that 1 2 A. Then m = + + ::: + 2 A. Thus, m 2 A n n n n n n | {z } m times 1 if and only if n 2 A. 1 Theorem 1.2: For relatively prime m and n, the rational number mn 2 A if 1 1 and only if m 2 A and n 2 A. 1 1 1 Proof: First, assume that 1 2 A. So then 1 = m = + + ::: + 2 mn n mn mn mn mn | {z } m times 1 1 1 A. Similarly, 1 = n = + + ::: + 2 A. m mn mn mn mn | {z } n times 1 1 1 1 1 1 Now, assume that m 2 A and n 2 A. Then m + n 2 A. So m + n = n m m+n mn + mn = mn 2 A. Since m and n are relatively prime, m + n and mn are 4 1 1 relatively prime. Thus, by Theorem 1.1, mn 2 A. And so mn 2 A if and only 1 1 if m 2 A and n 2 A. These two theorems give us some tools to deduce what certain elements of a subgroup will look like given one element. Theorem 1.1 emphasizes that it is the denominators of elements that are definitive of a subgroup's structure. This will be important when we attempt to completely characterize a group in a simple way. Theorem 1.2 is useful because it allows us to focus on the elements of a group based on the primes that make up the denominators of the elements. Decomposing the denominator of an element into it's prime factorization puts the element into a form that we can apply Theorem 1.2 to, and subsequently look at elements whose denominators are primes. As an example, let A be a 3 1 1 unitary subgroup of Q. If 10 2 A, then 10 2 A, and furthermore 2 2 A and 1 5 2 A. m In general, for a unitary subgroup A, if n 2 A with m and n relatively e1 ej prime, and n = p1 ∗ ::: ∗ pj is the prime factorization of n, then each of the 1 1 ej e1 ;:::; are in A. p1 pj Having given some thought to the form and nature of subgroup elements, we will now turn our attention to the homomorphisms and, in turn, isomorphisms of subgroups of the rationals. Theorem 1.3: For q 2 Q, f(x) = qx is a homomorphism. Proof: Let q 2 Q and let f(x) = qx. For any a; b 2 A, f(a + b) = q(a + b) = qa + qb = f(a) + f(b). So f is a homomorphism. Related to Theorem 1.3, Theorem 1.4: If f is any homomorphism from a subgroup A to Q (A need not be unitary), then there is a unique q such that f(x) = qx. p p m Proof: Let f : A ! Q be a homomorphism. For q 2 A, let f( q ) = n . Now, p we multiply both sides of a = q by mq, so that amq = pm. Now, multiply- 5 p m p ing both sides of f( q ) = n by pn, we get that f( q )pn = mp = amq so then p mq mq f( q ) = pn ∗ a. Let g(x) = pn ∗ x. We want to show that f = g, but we need first to prove a quick lemma. Lemma 1.4.1: If f; g : A ! Q are homomorphisms and for some non-zero a 2 A we have that f(a) = g(a).

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