Homotopy groups of quasi-spheres and applications to indefinite orthogonal groups

Abstract

In this note, we present a new proof of the isomorphism $\pi_1(SO^+(p,q)) \cong \pi_1(SO(p))\times \pi_1(SO(q))$ using the long exact sequence associated to a fibration. While this formula is already known, the method of proof presented here fills an existing hole in the literature to naturally generalize an approach following the classical reduction of this formula for $q=0$, involving the homotopy groups of the $(p-1)$-sphere and a long exact sequence arising from a fibration constructed from these spaces. To generalize for $q \ne 0$, we upgrade spheres to the corresponding quasi-spheres, then analyze the resulting long exact sequence to obtain the isomorphism for all $p,q$. Some low-dimensional inductive steps are delicate in this approach, but it is more direct than other standard methods and involves some interesting calculations with fundamental groups. The question of whether or not this approach would work is absolutely natural, and in this paper we show that the answer is yes. This particular approach avoids some difficult technical steps in other standard proofs, which we review at the end. The family of spaces we consider are also of interest in physical applications. We remark about this direction to provide some context, but do not explore it in depth in this paper.