An uncertainty principle for convolution operators on discrete groups

Abstract

Consider a discrete group $G$ and a bounded self-adjoint convolution operator $T$ on $l^{2}(G)$; let $\sigma (T)$ be the spectrum of $T$. The spectral theorem gives a unitary isomorphism $U$ between $l^{2}(G)$ and a direct sum $\bigoplus _{n} L^{2}(\Delta _{n},\nu )$, where $\Delta _{n}\subset \sigma (T)$, and $\nu$ is a regular Borel measure supported on $\sigma (T)$. Through this isomorphism $T$ corresponds to multiplication by the identity function on each summand. We prove that a nonzero function $f\in l^{2}(G)$ and its transform $Uf$ cannot be simultaneously concentrated on sets $V\subset G$, $W\subset \sigma (T)$ such that $\nu (W)$ and the cardinality of $V$ are both small. This can be regarded as an extension to this context of Heisenberg’s classical uncertainty principle.