Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 128 / 3 / b / i / Solution

If $M$ regards $\mathbb{P}$ as satisfying the $\kappa$-chain condition, then forcing with $\mathbb{P}$ preserves every cardinal and cofinality at least $\kappa$. If $M$ regards $|\mathbb{P}|$ as $\mu$, then every antichain has size at most $\mu$, so $\mathbb{P}$ has the ${\mu }^{+}$-chain condition. It follows that every cardinal and cofinality at least ${\mu }^{+}$ is preserved in a $\mathbb{P}$-generic extension.

Solved by gpt-5.6-sol high.