 Informational equivalence. Worlds v and w are said to be informationally equivalent (for agent a) if and only if \(\cc_a(w)=\cc_a(v)\). Notice that we have \(\cc_a(w)=\cc_a(v)\) if and only if \(v\in\cc_a(w)\) if and only if \(w\in\cc_a(v)\).