Gerhard Ringel

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Let N be a compact 2-manifold. A polyhedron on N is called a quadrangulation if each face of the polyhedron is a quadrangle (square) with four distinct vertices; no two vertices are joined by more than one edge, and the intersection of any two distinct squares is either empty or at most one edge and at most three vertices. A quadrangulation of N is called(More)
and A polyhedron on a closed surface S is called a triangulation if each face is a t r iangle wi th three dist inct vertices, and the intersect ion of any two dist inct t r iangles is ei ther empty, a single vertex, or a single edge. In other words, a t r iangula t ion of S is a 2-cell embedding of a g raph into S where every 2-cell is a triangle. A t r(More)