Commensurable ordinals
= Commensurable ordinals
Two <ordinals> are commensurable when neither is absorbed by adding the other on its left. For positive ordinals, this is equivalent to equality of their leading <Cantor normal form> exponent. If leading exponents differ, the lower-leading ordinal is absorbed by the higher power of $\omega$. If they agree, either sum adds the two positive leading coefficients and therefore exceeds both original ordinals. Zero is commensurable with no ordinal.