  It is shown that systems described by Harper's equation exhibit a Dirac point
at the center of the spectrum whenever the field parameter is a fraction of
even denominator. The Dirac point is formed by the touching of two subbands,
and the physics around such point is characterized by the relative field only,
as if the latter were null at the reference value. Such behavior is consistent
with the nesting property conjectured by Hofstadter, and its experimental
verification would give support to such hypothesis as well as the
Peierls-Onsager ansatz used to arrive at Harper's equation when crystalline
electrons move in a uniform external magnetic field
