  We use Lee-Suzuki mappings and related techniques to construct effective
two-body p-shell interactions and neutrinoless double-beta operators that
exactly reproduce the results of large no-core-shell-model calculations of
double-beta decay in nuclei with mass number A=6. We then apply the effective
operators to the decay of nuclei with A=7, 8, and 10, again comparing with
no-core calculations in much larger spaces. The results with the effective
two-body operators are generally good. In some cases, however, they differ
non-negligibly from the full no-core results, suggesting that three-body
corrections to the decay operator in heavier nuclei may be important. An
application of our procedure and related ideas to fp-shell nuclei such as 76Ge
should be feasible within coupled-cluster theory.
