Following Parreau’s work in
1951-52, we give a unified definition of parabolic Riemann surfaces, with
or without boundary. A surface is parabolic under the unified definition
implies that it is either relative parabolic or parabolic under the classical
definitions.
Then we study the conformal structures of noncompact, proper, branched
minimal surfaces in ℝ3 and prove several criteria of such surfaces (with or without
boundary) being parabolic. Using these criteria we then prove two graph theorems,
they are noncompact versions of the classical graph theorem of Radó, generalized in
various directions.