Suppose we have a finite connected graph $G$, I want to add 2 -cells to $G$ so that the 2 cells have boundaries of length 4 (squares) and so that $G$ is the 1 skeleton of a surface (2-manifold) without boundary. To check that such a process can be done to $G$ does it suffice to check whether each edge in $G$ occurs as an edge of 2 distinct 4 -cycles? does it suffice to check if each edge occurs as exactly 2 distinct 4 cycles? Is there a known sufficient condition?

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    $\begingroup$ No, that won't suffice. What you need to check is primarily at the vertices. If a 2-cell is incident to a vertex, it gives an ordering of the two edges that it is incident to at that vertex. What you need is that this gives a cyclic ordering of the edges incident. In principle, you could have a non-cyclic ordering, for example, you could have two disjoint cycles. This is what you'd get if you wedged two planar graphs together along a common vertex. $\endgroup$ – Ryan Budney Mar 26 '13 at 21:18
  • $\begingroup$ @Ryan, Do you know of a sufficient condition so that this process can be done? $\endgroup$ – Antony Della Vecchia Mar 26 '13 at 21:40
  • $\begingroup$ What I mentioned is a necessary and sufficient condition. The edge condition that you mention is a consequence of the vertex condition I describe in my original comment. $\endgroup$ – Ryan Budney Mar 26 '13 at 21:44

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