Multipliers in the scale of periodic Bessel potential spaces with smoothness indices of different signs

Abstract

We prove a general type description result for the multipliers acting between two periodic Bessel potential spaces, defined on the $n$--dimensional torus, in a case when their smoothness indices are of different signs. This is done through the detailed examination of a periodic analogue of the linear operator $J_s$, which is employed in the definition of the scale of the Bessel potential space defined on the whole space $\mathbb{R}^n$. Our method of defining this periodic analogue of $J_s$ uses the results about an asymptotic behaviour of the generalized Fourier coefficients and existence of a natural homeomorphism between the spaces $\mathcal{D}'(\mathbb{T}^n)$ and $S'_{2 \cdot \pi}(\mathbb{R}^n)$, where the latter consists of all $2 \cdot \pi$--periodic distributions from the dual Schwartz space $\mathcal{S}'(\mathbb{R}^n)$.