EN
Let n ≥ 2 and $H_k^{s,s'} = {u∈ S'(ℝ^n): ∥u∥_{s,s'} < ∞}$, where
$∥u∥²_{s,s'} = (2π)^{-n} ∫(1+|ξ|²)^s (1+|ξ'|²)^{s'}|Fu(ξ)|²dξ $,
$Fu(ξ) = ∫e^{-ixξ} u(x) dx$, $ξ'∈ ℝ^k$, k < n. We prove that for some s,s' the space $H^{s,s'}_k$ is a multiplicative algebra.