On the product of periodic distributions. Product in shift-invariant spaces

We connect through the Fourier transform shift-invariant Sobolev type spaces V_s⊂ H^s, s∈ℝ, and the spaces of periodic distributions and analyze the properties of elements in such spaces with respect to the product. If the series expansions of two periodic distributions have compatible coefficient estimates, then their product is a periodic tempered distribution. We connect product of tempered distributions with the product of shift-invariant elements of V_s. The idea for the analysis of products comes from the Hörmander's description of the Sobolev type wave front in connection with the product of distributions. Coefficient compatibility for the product of f and g in the case of "good" position of their Sobolev type wave fronts is proved in the 2-dimensional case. For larger dimension it is an open problem because of the difficulties on the description of the intersection of cones in dimension d⩾3.