Rational Hopf G-spaces with two nontrivial homotopy group systems

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1 F U N D A M E N T A MATHEMATICAE 146 (1995) Rational Hopf -spaces with two nontrivial homotopy group systems by Ryszard D o m a n (Poznań) Abstract. Let be a finite group. We prove that every rational -connected Hopf -space with two nontrivial homotopy group systems is -homotopy equivalent to an infinite loop -space. 1. Introduction. It is known that a rational H-space X of the homotopy type of a connected CW -complex is homotopy equivalent to a weak product of Eilenberg MacLane spaces [3], and thus to an infinite loop space. In this paper we study the question of whether there is an analogous result for X admitting a finite group action compatible with the H-structure. Let be a finite group. -spaces, -maps and -homotopies considered in this paper will be pointed. We shall work in the category of -spaces having the -homotopy type of -CW -complexes [1]. In case of need, we shall tacitly replace -spaces by their -CW -substitutes. We shall also assume all -spaces to be -connected in the sense that all the fixed point spaces X H are connected for all subgroups H of. Definition. A Hopf -space is a Hopf space X on which acts in such a way that the multiplication m : X X X is -equivariant, and the composite X X X X m X is -homotopic to the folding map. For example, if Y is a -space, then the loop space ΩY is a Hopf -space, where the action of is defined by (gf)(t) = g(f(t)). Let X be a -simple -space (i.e. each X H is simple). We shall call X rational if the homotopy groups π i (X H ) are Q-vector spaces for each subgroup H of. Note that, by [4], every -simple -space, in particular Hopf -space, can be rationalized. Moreover, in the latter case, the resulting -space is a Hopf -space Mathematics Subject Classification: Primary 55P45, 55P62, 55P91. Supported by Polish scientific grant RP.I.10. [101]

2 102 R. Doman Let O be the category of canonical orbits of and -maps between them. A coefficient system for is a contravariant functor from O to the category of abelian groups. We shall call a coefficient system rational if its range is the category of Q-vector spaces. For a -space X, the homotopy and homology group systems π n (X) and H n (X) are defined by π n (X)(/H) = π n (X H ), Hn (X)(/H) = H n (X H ), where H n denotes the reduced singular homology with Z-coefficients. iven a coefficient system M for and an integer m 1, an Eilenberg MacLane -space of type (M, n) is a -space X such that π m (X) = M and π q (X) = 0 for q m. Such -spaces always exist and are unique up to -homotopy equivalence [1]. We shall call a -space X 0 an infinite loop -space if there exist a sequence X 0, X 1, X 2,... of -spaces and a sequence f n : X n ΩX n+1, n 0, of -homotopy equivalences. Eilenberg MacLane -spaces ere examples of infinite loop -spaces. Moreover, using the -obstruction argument [1], it can be shown that a -Hopf structure on an Eilenberg MacLane -space is unique up to - homotopy. We shall denote an Eilenberg MacLane -space of type (M, m) by K(M, m). Recall also that Eilenberg MacLane -spaces represent Bredon cohomology [1]: H m (X, M) = [X, K(M, m)], where [, ] denotes the -homotopy classes of -maps. The relation between H m (X, M) and H (X) is given by a spectral sequence with E p,q 2 = Ext p ( H p+q q (X), M) H (X, M) [1]. We shall refer to it as the Bredon spectral sequence. Let Z/p k denote the cyclic group of order p k, where p is prime and k is a positive integer. A theorem by. Triantafillou [5] states that each rational Z/p k -connected Hopf Z/p k -space is Z/p k -homotopy equivalent to a weak product of Eilenberg MacLane Z/p k -spaces, and hence to an infinite loop Z/p k -space. In contrast to the nonequivariant case, however, rational Hopf -spaces do not split equivariantly into a product of Eilenberg MacLane -spaces in general [5]. Nevertheless, the counterexamples given in [5] are still infinite loop -spaces. This gives rise to the question of whether every rational Hopf -space X is -homotopy equivalent to an infinite loop - space. In the present paper we answer the above question affirmatively in the case where X has only two nontrivial homotopy group systems. Thus we prove the following Theorem. Let X be a rational -connected Hopf -space having only two nontrivial homotopy group systems. Then X is -homotopy equivalent to an infinite loop -space.

3 Rational Hopf -spaces Equivariant k-invariants of Hopf -spaces. Let X be a Hopf -space with a -multiplication m : X X X and let N be a coefficient system for. An element u of Hn (X, N) is called primitive if m (u) = p 1(u) + p 2(u) in H n (X X, N), where p 1 and p 2 are the two projections. Now, let be an equivariant Postnikov decomposition of X (see [4]). We shall use the following results of [5]: Proposition 2.1. Each X n is a Hopf -space. Proposition 2.2. The equivariant k-invariant k n+1 H n (X n 1, π n (X)) is primitive for all n Primitive elements in the Bredon cohomology of rational Eilenberg MacLane -spaces. For a Hopf -space Y with a -multiplication µ : Y Y Y and a coefficient system N for, consider the homomorphism t = µ p 1 p 2 : Hn (Y, N) H n (Y Y, N), where p 1 and p 2 are the two projections. Let H n (Y, N) = J 0,n... J n,0 = 0 and H n (Y Y, N) = F 0,n... F n,0 = 0 be the filtrations corresponding to the Bredon spectral sequences converging to H n (Y, N) and H n (Y Y, N), respectively. By our assumptions about -spaces, the - cellular approximation theorem [1], and the construction of the Bredon spectral sequence, the homomorphism t preserves the filtrations, and the induced homomorphism E p,q (Y ) E p,q (Y Y ) can be identified with the one induced by Ext p (t, N) : Ext p ( H q (Y ), N) Ext p ( H q (Y Y ), N), where t = µ p 1 p 2 : H q (Y Y ) H q (Y ) is a morphism of coefficient systems. We can summarize the above in Proposition 3.1. The homomorphism t : H n (Y, N) H n (Y Y, N) is the limit of a morphism of the Bredon spectral sequences whose E p,q 2 -term is Ext p (t, N) : Ext p ( H q (Y ), N) Ext p ( H q (Y Y ), N). We are now going to examine the morphism t : Hq (Y Y ) H q (Y ) for Y being a rational Eilenberg MacLane -space. Proposition 3.2. Let Y be an Eilenberg MacLane -space of type (M, m). Then the morphism t : Hq (Y Y ) H q (Y ) has a right inverse for each q m.

4 104 R. Doman P r o o f. For each subgroup H of, the fixed point space Y H is a rational Eilenberg MacLane -space of type (M(/H), m). Thus, by [2, Appendix], the Pontryagin algebra H (Y H ) is the free graded commutative algebra generated by H m (Y H ). In particular, the multiplication µ H : H (Y H ) H (Y H ) H (Y H ) is a graded algebra homomorphism. Now suppose that a 1,..., a k belong to H m (Y H ), and let a 1... a k H km (Y H ) be their product. Let H : Y H Y H Y H be the diagonal map. Since every element of H m (Y H ) is primitive, we have (µ H p H 1 p H 2 ) H (a 1... a k ) = µ H H (a 1... a k ) 2a 1... a k = µ H ((a a 1 )... (a k a k )) 2a 1... a k = (2 k 2)a 1... a k. This implies that (1/(2 k 2)) : Hkm (Y ) H km (Y Y ), where : Y Y Y is the diagonal, is a right inverse of t for each k 1. Since H q (Y ) = H q (Y Y ) = 0 for q km, the desired result follows. For each element u H n (Y, N), define the weight w(u) of u to be the greatest lower bound of the integers q such that u J n q,q, where H n (Y, N) = J 0,n... J n,0 = 0 is the filtration corresponding to the Bredon spectral sequence. Proposition 3.3. Suppose that Y is an Eilenberg MacLane -space of type (M, m), where M is a rational coefficient system for. Then w(u) m for every primitive element u of H n (Y, N). P r o o f. Let {J p,n p (Y )} and {J p,n p (Y Y )} be the filtrations of H n (Y, N) and H n (Y Y ), respectively, which correspond to the Bredon spectral sequences. Suppose that u H n (Y, N) is primitive and set w(u) = q. Consider the commutative diagram J n q,q (Y ) α γ E n q,q β (Y ) J n q,q (Y Y ) E n q,q (Y Y ) where α is the restriction of t : H n (Y, N) H n (Y Y ), β is induced by Ext n q (t, N) : Ext n q ( H q (Y ), N) Ext n q ( H q (Y Y ), N), and γ is the projection. If w(u) > m then, by Proposition 3.2, β is a monomorphism. Thus βγ(u) 0. Consequently, u cannot be primitive. 4. Proof of Theorem. Let X be a rational Hopf -space having only two nontrivial homotopy group systems π m (X) = M and π n (X) = N, m < n. Then X is determined by its equivariant k-invariantk(x)

5 H n+1 Rational Hopf -spaces 105 (K(M, m), N), which, by Proposition 2.2, is primitive. The cohomology suspension σ q+1 : H (K(M, r + 1), N) H q (K(M, r), N), which corresponds to the map Ω : [K(M, r+1), K(N, q+1)] [K(M, r), K(N, q)], is, by Lemma 3.3 of [5], the limit of a morphism of spectral sequences with E 2 -term Ext i (σ, N) : Ext i ( H j+1 (K(M, r + 1)), N) Ext i ( H j (K(M, r)), N), where σ : Hj (K(M, r)) H j+1 (K(M, r + 1)) is determined by homology suspension. In order to prove the Theorem we only need to show that the equivariant k-invariant k(x) belongs to the image of the composite H n+k n+k 1 (K(M, m + k 1), N) H (K(M, m + k 2), N)... n+1 H (K(M, m), N) of cohomology suspensions for each k > 1. By Proposition 3.3, we know that w(k(x)) m. Thus the proof of the Theorem will be completed if we prove the following q+1 Proposition 4.1. Let H (K(M, r+1), N) = F 0,q+1... F q+1,0 = 0 and H q (K(M, r), N) = J 0,q... J q+1,0 = 0 be the filtrations corresponding to the Bredon spectral sequences, where q n+1 and r = m+q n 1. Then the cohomology suspension σ : Hq+1 (K(M, r + 1), N) H q (K(M, r), N) restricted to F q r,r+1 gives an isomorphism σ : F q r,r+1 J q r,r. P r o o f. Denote by E, the Bredon spectral sequence converging to H q+1 (K(M, r), N), and by E, the one converging to H q (K(M, r), N). We have and Hence E q r,r+1 E q 1,r+1 2 = Ext q 1 ( H r+1 (K(M, r + 1)), N) E q r,r 2 = Ext q r ( H r+1 (K(M, r)), N). = F q r,r+1 and E q r,r = J q r,r. Under the above identification, σ is induced by σ : Hr (K(M, r)) H r+1 (K(M, r + 1)). Since, evidently, σ is an isomorphism, so is σ. R e m a r k 4.2. Since we have not used the assumption that the coefficient system N is rational, the conclusion of the Theorem is valid for a Hopf - space X having only two nontrivial homotopy group systems π m (X) and π n (X), m < n, with π m (X) rational.

6 106 R. Doman References [1]. E. Bredon, Equivariant Cohomology Theories, Lecture Notes in Math. 34, Springer, [2] J. W. Milnor and J. C. Moore, On the structure of Hopf algebras, Ann. of Math. 81 (1965), [3] H. Scheerer, On rationalized H- and co-h-spaces, Manuscripta Math. 51 (1984), [4]. W. Triantafillou, Equivariant minimal models, Trans. Amer. Math. Soc. 274 (1982), [5], Rationalization of Hopf -spaces, Math. Z. 182 (1983), FACULTY OF MATHEMATICS AND COMPUTER SCIENCE ADAM MICKIEWICZ UNIVERSITY MATEJKI 48/ POZNAŃ, POLAND Received 2 September 1992; in revised form 15 March 1994