On the Mislin Genus of Symplectic Groups

Abstract

In this paper we give lower bounds for the Mislin genus of the symplectic groups $\mathrm{Sp}(m)$. This result appears to be the exact analogue of Zabrodsky's theorem concerning the special unitary groups $\mathrm{SU}(n)$ . It is achieved by the determination of the stable genus of the quasi-projective quaternionic spaces $Q\mathbb{H}(m)$ , following the approach of McGibbon. It leads to a symplectic version of Zabrodsky's conjecture, saying that these lower bounds are in fact the exact cardinality of the genus sets. The genus of $\mathrm{Sp}(2)$ is well known to contain exactly two elements. We show that the genus of $\mathrm{Sp}(3)$ has exactly 32 elements and see that the conjecture is true in these two cases.

Independently, we also show that any homotopy type in the genus of $\mathrm{Sp}(m)$ fibers over the sphere $S^{4m-1}$ with fiber in the genus of $\mathrm{Sp}(m-1)$ , and that any homotopy type in the genus of $\mathrm{SU}(n)$ fibers over the sphere $S^{2n-1}$ with fiber in the genus of $\mathrm{SU}(n-1)$ . Moreover, these fibrations are principal with respect to some appropriate loop structures on the fibers. These constructions permit us to produce particular spaces realizing the lower bounds obtained.