User blog:Kyodaisuu/Pseudo Rayo's number

Pseudo Rayo's number is the smallest positive integer that cannot be named by an expression in the language of first order set theory (FOST) with a googol symbol or less. As FOST has 17 symbols, 1234567890∈=¬∧∃, it is smaller than \(17^{10^{100}}\). It is not a very big number in googology, but it is very difficult to express its exact value without using this expression.