letLet $S$ be an infinite dimensional sphere in a Hillbert space.
As $S$ is homotopic to the product $S \times S$, then $S$ is diffeomorphic to $S \times S$ (For hillbertfor Hilbert manifolds, homotopica homotopy equivalence implies diffeomorphism).
Is there an explicit diffeomorphism between $S$ and $S \times S$?