18. Graph colourings
On the bank of the river he saw a tall tree: from roots to crown one half was aflame and the other green with leaves.
‘Peredur son of Evrawg’from The Mabinogion (earlier than 1325)
TOPICS: Vertex and edge colourings; perfect graphs; graph minors; embeddings of graphs in surfaces
TECHNIQUES: Use of Max-Flow Min-Cut Theorem for constructions; alternating chain arguments