Figure 6. Graph P obtained by identifying red and blue points respectively is a digital 2-dimensional projective plane. The join S0(x,y)⊕P is the digital 3-surface with a local non-spherical topology. The rims O(x) and O(y) of points x and y are both a digital 2-dimensional projective plane P. To make this operation visible, the join of a continuous projective plane P and S0(x,y) is shown

From

Graph Theoretical Models of Discrete Spaces with Locally Non-spherical Topology

Alexander V. Evako

Applied Mathematics and Physics. 2017, 5(2), 47-52 doi:10.12691/amp-5-2-3