Fill in each circle with a letter so that the two networks are identical; that is, if two letters have a line connecting them in one network, then those two letters have a line connecting them in the other network, and vice versa.
(Rules and example from PGP IB)
History of the puzzle
First appeared on WPC 2002/Part 5. The puzzle was written by Olga Leontieva.
In terms of graph theory, this puzzle is about finding a unique graph isomorphism between the given two graphs.