Reductions for Erdős Problem 919
Erdős Problem 919 asks for a graph on the ordinal omega_2 squared with chromatic number aleph_2, or aleph_1, such that every subgraph on a vertex set of smaller order type has chromatic number at most aleph_0. This formalization proves in ZFC that the aleph_2 question is equivalent to the first question of Erdős Problem 918, that the aleph_1 question is equivalent to the existence of a graph on aleph_2 vertices that is not countably chromatic although all its subgraphs on aleph_1 vertices are, that the aleph_2 question implies the aleph_1 question, and that the aleph_1 question has a…
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