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Mahlo cardinal

In mathematics, a cardinal number κ > ‭א‬0 is called Mahlo iff the set U = {λ < κ: λ is inaccessible} is stationary in κ. Assuming that ZFC is consistent, the existence of Mahlo cardinals provably cannot be proved in ZFC.

Mahlo cardinals were first described in 1911 by mathematician Paul Mahlo.