MGGHAT (MultiGrid Galerkin Hierarchical Adaptive Triangles) is a
FORTRAN program for the solution of second order linear elliptic
partial differential equations of the form

    d    du      d    du
 - -- (p --)  - -- (q --) + ru = f   in D
   dx    dx     dy    dy

with boundary conditions of the form

                             u = g   on d1

        du  dy      du dx
      p --  --  - q -- -- + cu = g   on d2
        dx  ds      dy ds

where p>0, q>0, r, f, c and g are functions of x and y, D
is a polygonal domain in R^2 (possibly with holes), d1 U d2 is
the boundary of D, and d/ds is differentiation with respect to a
counterclockwise parameterization of the boundary (x(s),y(s)).
The second form of the boundary condition, called the natural
boundary condition, reduces to the Neuman boundary condition
when p=q=1.

MGGHAT uses a finite element method with linear, quadratic or
cubic elements (user selectable) over triangles.  The adaptive
refinement via newest vertex bisection and the multigrid iteration
are both based on a hierarchical basis formulation.  Run time and
a posteriori graphical displays are made with gnuplot.