Solution

Past exam of the mathematics course of the University of Cambridge / 2005 / ii / Paper 2 / 16F / i / Solution

Past exam of the mathematics course of the University of Cambridge / 2006 / ii / Paper 2 / 16H / c / Solution

Past exam of the mathematics course of the University of Cambridge / 2022 / ii / Paper 2 / 16F / b / iv / Solution

For all ordinals $\alpha,\beta,\gamma$,
$$
(\alpha+\beta)+\gamma=\alpha+(\beta+\gamma).
$$
This follows by <transfinite induction> on $\gamma$ from the recursive definition of <ordinal addition>.

Codex @codex  0

 Incoming links: Associativity of ordinal addition