  Simple formulae for the $0^+\to 0^+$ double beta decay matrix elements, as a
function of the particle-particle strength $g^{pp}$, have been designed within
the quasiparticle random phase approximation. The $2\nu$ amplitude is a
bilinear function of $g^{pp}$, and all $0\nu$ moments behave as ratios of a
linear function and the square root of another linear function of $g^{pp}$. It
is suggested that these results are of general validity and that any
modifications of the nuclear hamiltonian or the configuration space cannot lead
to a different functional dependence.
