We prove that the p-completed Brown-Peterson spectrum is a retract of a product of Morava E-theory spectra. As a consequence, we generalize results of Ravenel-Wilson-Yagita and Kashiwabara from spaces to spectra and deduce that the notion of good group is determined by Brown-Peterson cohomology. Furthermore, we show that rational factorizations of the Morava E-theory of certain finite groups hold integrally up to bounded torsion with height-independent exponent, thereby lifting these factorizations to the rationalized Brown-Peterson cohomology of such groups.2010 Mathematics Subject Classification 55N20 (primary), 55N22, 55R40 (secondary) Keywords: Brown-Peterson spectrum, Morava E-theory, transchromatic character theory.
Brown-Peterson cohomology from Morava E-theoryTheorem. Let A be a finite abelian group, then the exponent of the torsion in the cokernel of the natural mapis bounded independent of the height n.In order to prove this, we construct a variant of the transchromatic character maps of [Sta13, BS16] from E-theory at height n to height 1, which allows for tight control over the torsion. Since products of torsion abelian groups with a common torsion exponent are torsion as well, it follows that the natural mapis a rational isomorphism. It is possible to apply the retract theorem to immediately deduce a similar decomposition for the BP -cohomology of finite abelian groups.Corollary. Let A be a finite abelian group and let I denote the transfer ideal, then the natural mapSimilarly, the scheme corepresented by E * n (BΣ m ) decomposes rationally into a product of subgroup schemes Γ Sub λ⊢m (G En ), indexed by partitions λ ⊢ m of m. Again, we prove that this statement holds integrally up to globally bounded integral torsion.Theorem. The exponent of the torsion in the cokernel of the natural mapis bounded independent of the height n. Consequently, there is a rational isomorphismwhere I λ is a certain transfer ideal.Finally, we give a further illustration of the methods of the paper by proving a well-known version of Artin induction for the BP -cohomology of good groups.
Relation to the literatureThe question of when and how E * (X) can be computed algebraically from BP * (X) or, conversely, what kind of information about BP * (X) is retained in E * (X) has a long history. After pioneering work of Johnson and Wilson [JW73, JW75] and Landweber [Lan76], these problems have been studied systematically in a series of papers by Ravenel, Wilson, and Yagita [RWY98, Wil99] and Kashiwabara [Kas98, Kas01]. Their methods are based on a careful study of the associated Atiyah-Hirzebruch spectral sequences. As a consequence, the results contained in these papers are unstable, i.e., only valid for spaces rather than arbitrary spectra.The results of our paper generalize the main structural theorems of [RWY98, Wil99] from spaces to spectra, by replacing the Atiyah-Hirzebruch spectral sequence arguments by the above