The Gromov-Hausdorff distance between spheres

We provide general upper and lower bounds for the Gromov-Hausdorff distance $d_{\mathrm{GH}}(\mathbb{S}^m,\mathbb{S}^n)$ between spheres $\mathbb{S}^m$ and $\mathbb{S}^n$ (endowed with the round metric) for $0\leq m< n\leq \infty$. Some of these lower bounds are based on certain topological ideas related to the Borsuk-Ulam theorem. Via explicit constructions of (optimal) correspondences we prove that our lower bounds are tight in the cases of $d_{\mathrm{GH}}(\mathbb{S}^0,\mathbb{S}^n)$, $d_{\mathrm{GH}}(\mathbb{S}^m,\mathbb{S}^\infty)$, $d_{\mathrm{GH}}(\mathbb{S}^1,\mathbb{S}^2)$, $d_{\mathrm{GH}}(\mathbb{S}^1,\mathbb{S}^3)$ and $d_{\mathrm{GH}}(\mathbb{S}^2,\mathbb{S}^3)$. We also formulate a number of open questions.Comment: * We added an alternative correspondence between S^1 and S^2 with the optimal distortion in Appendix