Neutrinoless Double $\beta$-Decay:The Problem of Nuclear Matrix Elements
  The calculations of nuclear matrix elements of $0\nu\beta\beta$-decay is a challenge for nuclear physics. We are discussing here a model independent method, which could allow to test the calculations. The method is based on the factorization property of the nuclear matrix elements and requires observation of neutrinoless double $\beta$-decay of several nuclei. 
S. M. Bilenky