Relation between the $2\nu\beta\beta$ and $0\nu\beta\beta$ nuclear   matrix elements
  A formal relation between the GT part of the nuclear matrix elements $M^0\nu}_{GT}$ of $0\nu\beta\beta$ decay and the closure matrix elements $M^{2\nu}_{cl}$ of $2\nu\beta\beta$ decay is established. This relation is based on the integral representation of these quantities in terms of their dependence on the distance $r$ between the two nucleons undergoing transformation. We also discuss the difficulties in determining the correct values of the closure $2\nu\beta\beta$ decay matrix elements. 
Petr Vogel and Fedor \v{S}imkovic
nucl-th hep-ph