Product of octahedra is badly approximated in the $\ell_{2,1}$-metric

Abstract

We prove that the cartesian product of octahedra $B_{1,\infty}^{n,m}=B_1^n\times\ldots\times B_1^n$ ($m$ octahedra) is badly approximated by half--dimensional subspaces in mixed--norm: $d_{N/2}(B_{1,\infty}^{n,m},\ell_{2,1}^{n,m})\ge cm$, $N=mn$. As a corollary the orders for linear widths of H\"older--Nikolskii classes $H^r_p(\mathbb T^d)$ in the $L_q$ metric are obtained for $(p,q)$ in a certain set (a domain in the parameter space).