Sampling of Band-Limited Vectors.
Let be the Heisenberg group of dimension . Let be the partial sub-Laplacians on and the central element of the Lie algebra of . We prove that the kernel of the operator is in the Schwartz space if . We prove also that the kernel of the operator is in if and that the kernel of the operator is in if . Here is the Kohn-Laplacian on .
A measure is called -improving if it acts by convolution as a bounded operator from to L² for some q < 2. Interesting examples include Riesz product measures, Cantor measures and certain measures on curves. We show that equicontractive, self-similar measures are -improving if and only if they satisfy a suitable linear independence property. Certain self-affine measures are also seen to be -improving.