Did you know that two different graphs with 81 vertices and 20 edges/vertex are famous enough to have Wikipedia articles?

The strongly regular Brouwer–Haemers graph ( connects elements of GF(81) that differ by a fourth power.

The Sudoku graph ( connects cells of a Sudoku grid that should be unequal. Sudoku puzzles are instances of precoloring extension on this graph.

Unfortunately the natural graphs on the 81 cards of Set have degree ≠ 20...

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