Past exam of the mathematics course of the University of Cambridge / 2017 / iii / Paper 121 / 3 / ii / b

Conversely, suppose $p\in G$ and $p{⊩}^{\ast }\sigma \in \tau$. The syntactic forcing relation says thatis dense below a forcing condition $p$. This is a set in $M$ by definability of forcing and axiom schema of separation. The dense-below generic meeting lemma gives $s\in G\cap D$: augment $D$ by all incompatible forcing conditions with $p$ to obtain a globally dense ground-model set, and use $p\in G$ to exclude the incompatible part.

Choose the witnessing $(\rho ,r)\in \tau$. Since $s\le r$ and $s\in G$, upward closure gives $r\in G$. The assumed equality truth lemma gives ${\sigma }^G={\rho }^G$, while $r\in G$ gives ${\rho }^G\in {\tau }^G$. Combining both directions,This completes the atomic membership truth lemma for forcing; no separate assumption of the membership truth lemma was made.