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A 4-cycle, which denotes the graph below,

*---------* | | | | | | *---------*has the following two properties:

(1) If two points are connected, they have no common neighbors.

(2) If two points are not connected, they have exactly two common neighbors.Find a graph with more than four vertices that has these two properties.

© Copyright 1999 Stan Wagon. Reproduced with permission.

19 October 1999