PROCEEDINGS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 81, Number 2, February 1981 GROUP RINGS WHOSE TORSION UNITS FORM A SUBGROUP CÉSAR POLCINO MILIES1 Abstract. Denote by TU(ZG) the set of units of finite order of the integral group ring of a group G. We determine the class of all groups G such that TUÇLG) is a subgroup and study how this property relates to certain properties of the unit groups. 1. Introduction. Let G be a group. We denote by ZG its integral group ring and by i/(ZG) the group of units of this ring. Also, we shall denote by T = T(G) and TU(ZG) the set of all elements of finite order in G and U(ZG) respectively. S. K. Sehgal and H. J. Zassenhaus have determined the classes of all groups G such that U(ZG) is a nilpotent or an FC group in [3] and [4]. If restricted to finite groups, it is easy to see that both classes coincide. M. M. Parmenter and C. Polcino Milies have shown in [1] that, in the finite case, the characterization of such groups G follows from the fact that, in both cases, TU(ZG) forms a subgroup. In this note, we determine all groups G which are either nilpotent or FC and such that TU(ZG) is a subgroup. It will follow that this property is not enough to lead either to nilpotency or the FC property when G is infinite. 2. The Theorem. We begin with Lemma. Let G be a group such that TU(ZG) forms a subgroup. Then TU(ZG) ±T, i.e. every unit of finite order is trivial. — Proof. Let u E TU(ZG) with o(u) = p"' . . .pt\ We shall show that u E ±T by induction on t. If / = 1 then m is a p-element for some rational prime p; hence, there is an element g E supp(w) such that o(g) is finite [2, Theorem VI.2.1]. Since TU(ZG) is a subgroup, v = g~x • u is a unit of finite order and, writing v = "2aeGv(a)a, we have that t>(l) ¥= 0. It follows from [2, Corollary II.1.2] that v = ±1, thus u = ± g E T. Now, we assume that the result holds if o(u) is divisible at most by t — 1 different primes and let o(u) = p"' . . .p?. Received by the editors October 26, 1979. AMS (MOS) subject classifications(1970). Primary 16A26; Secondary 20C07. Key words and phrases. Group rings, unit groups, torsion units, nilpotent, FC group. 'This work was partially supported by the Conselho Nacional de Desenvolvimento Tecnológico (CNPq) from Brazil. © 1981 American 172 Científico e Mathematical 0002-9939/81/OOOO-O053/$Ol.75 License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use Society 173 GROUP RINGS Writing o(u) = mp,"1where gcd(m,pp) = 1, there exist integers r, s such that m + sp,"' = 1. Set ux = um and u2 = uv"'. Then u — uxu2 and «,, u2 G ±T by the induction hypothesis. □ We can now prove our main result. Theorem. Let G be a group such that TU(ZG) is a subgroup of U(ZG). Then T is a subgroup of G and one of the following conditions holds. (i) T is abelian and, for all x E G and all t E T, we have that x~xtx = t', where i = i(x) locally. (ii) T = Ks X E where Ks is the quaternion group of order 8 and E is an elementary abelian 2-group. Furthermore, E is central in G and conjugation by a fixed element of G induces in Ks one of its four inner automorphisms. Conversely, if G is a nilpotent or FC group satisfying (i) or (ii), then TU(ZG) is a subgroup of U(ZG). Proof. First assume TU(ZG) = ±T, hence T = Ks X E as in (ii). For elements t E T, + . . . +/<*>-!, Then u~x = 1 - (/ - that TU(ZG) is a subgroup. It follows from the Lemma that [1] or [2, Theorem II.4.1] shows that Fis either abelian or and x E G we consider u = 1 + (t — l)xt where t = I + t \)xt and utu~x - / + xt -2txt + t2xt. Since utu~x G TU(ZT) we must have txt = xt' or txt = t2xt' and consequently r*e</>. If T is abelian, the last equality shows that (i) holds. If T = Ks X E, it readily implies that E is central and that the automorphism induced in K8 is inner. Finally assume that either (i) or (ii) holds. From [2, Corollary VI. 1.24] we know that QG contains no nonzero nilpotent elements; hence, every idempotent is central and [2, Lemma VI.3.22] shows that U(ZG) = U(ZT) ■G. Now, we claim that if u E U(ZG) is an element of finite order, then u E U(ZT). Assume that u = vg where v E U(ZT) and g G G is an element of infinite order. Since gv = v'g for some v' E U(ZT), if n = o(u) we have that 1 = u" = v"g", withü" G U(ZT). Consequently g" G U(ZT), a contradiction. We have thus shown that TU(ZG) c (ZT). If (i) holds, U(Z T) is abelian and the result follows trivially. If (ii) holds, from [2, Corollary II.2.5] we have that U(ZT) = ±T, hence U(ZG) = ±G and thus TU(ZG) = ±T is a subgroup. \J As a consequence of the Theorem and a result of Sehgal [2, Theorem VI. 1.20] we have Corollary. Let G be a nilpotent or FC group such that TU(ZG) subgroup. Then ZG contains no nilpotent elements. forms a 3. An example. We conclude with an example showing that if G is infinite nilpotent or FC then the fact that TU(ZG) forms a subgroup does not imply that U(ZG) is either nilpotent or FC. License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 174 CÉSAR POLCINO MILIES Consider the group G = <r, x\t9 = 1, xtx ' = i7>. Denote (g„ gj) = gxg2gxxg2, Gm = <(g„g2)|g1;g2 G G> andGw = <(*, g)\x G G<n-'>,g G G>. It is easy to see that G(1) c </>. Now, if g = xktm we have (g, *») = x*/"x-*r" = i^7*-1) = t6n0t'l+7t'2+ ■■■ +1> g </3>. In a similar way it follows that G(3) = <1)- Thus G is nilpotent. Also, if C( g) denotes the conjugacy class of an element g G G, it is easy to see that C(g) c G(1)g = <i>g and is thus finite. Consequently, G is also an FC group. Hence, G satisfies the conditions in our theorem and is both nilpotent and FC, but it follows from Theorems VI.3.23 and VI.5.3 in [2] that U(ZG) is neither nilpotent nor FC. References 1. M. M. Paramenter and C. Polcino Milies, Group rings whose units form a nilpotent or FC group, Proc. Amer. Math. Soc. 68 (1978),247-248. 2. S. K. Sehgal, Topics in group rings, Dekker, New York, 1978. 3. S. K. Sehgal and H. J. Zassenhaus, Integral group rings with nilpotent unit groups, Comm. Algebra 5 (1977), 101-111. 4. _, Group rings whose units form an FC group, Math. Z. 153 (1977), 29-35. Instituto de Matemática e Estatistica, Iguatemi), Sâo Paulo, Brasil Untversidade de Sâo Paulo, Calxa Postal 20570 (Ao. License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use