NAME

       quadrangle - Quadrangular reference element

DESCRIPTION

       The quadrangular reference element is [-1,1]^2.

NUMBERING

             y

             3---------2
             |         |
             |         |
             |         |
             |         |
             0---------1   x

       Curved  high  order Pk quadrangles (k >= 1), in 2d or 3d geometries, are supported.  These
       quadrangles have additional edge-nodes and face-nodes.

THESE NODES ARE NUMBERED AS

        first vertex, then edge-node, following the edge numbering  order  and  orientation,  and
       finally  the  face  internal  nodes, following the quadrangle lattice.  See below for edge
       numbering and orientation.

             3-----6-----2   3---9---8---2
             |           |   |           |
             |           |  10  14  15   7
             7     8     5   |           |
             |           |  11  12  13   6
             |           |   |           |
             0-----4-----1   0---4---5---1
                   P2              P3

IMPLEMENTATION

       const size_t dimension = 2;
       const Float  measure = 4;
       const size_t n_vertex = 4;
       const point vertex [n_vertex] = {
               point(-1,-1),
               point( 1,-1),
               point( 1, 1),
               point(-1, 1) };
       const size_t  n_edge = 4;
       const size_t edge [n_edge][2] = {
               { 0, 1 },
               { 1, 2 },
               { 2, 3 },
               { 3, 0 } };