We have derived the popularly used parametrization formulae for quark masses
at low densities and modified them at high densities within the
mass-density-dependent model. The results are applied to investigate the lowest
density for the possible existence of strange quark matter at zero temperature.Comment: 9 pages, LATeX with ELSART style, one table, no figures. Improvement
  on the derivation of qark mass formula

Ning, P. Z.