Past exam of the mathematics course of the University of Cambridge / 2013 / iii / Paper 19 / 3 / ii / a / Solution

For every $\beta <{\omega }_1$, choose an injection $e_{\beta }:\beta \to \omega$. Define the Ulam matrix on omega-one byFor a fixed $\alpha$, every $\beta >\alpha$ belongs to exactly one of these sets, so their union is the entire tail $(\alpha ,{\omega }_1)$. Its complement is the countable ordinal $\alpha +1$. For distinct $\alpha ,{\alpha }^{\prime }$ and fixed $n$, membership in both sets would give $e_{\beta }(\alpha )=e_{\beta }({\alpha }^{\prime })=n$ for some $\beta$ above both, contradicting injectivity. This verifies both requested properties.