Isoparametric Hypersurfaces in Products of Simply Connected Space Forms

For $i\in\{1,2\}, $ let $\mathbb Q_{ε_i}^{n_i}$ denote the simply connected space form of dimension $n_i\ge 2$ and constant sectional curvature $ε_i\in\mathbb R$, $(ε_1,ε_2)\ne (0,0)$. We prove that any connected isoparametric hypersurface of $\mathbb Q_{ε_1}^{n_1}\times\mathbb Q_{ε_2}^{n_2}$ has co