Extensional Independence

Abstract

Joel Hamkins asks whether there is a $\Pi^0_1$-formula $\rho(x)$ such that $\rho(\phi)$ is independent over ${\sf PA}+\phi$, if this theory is consistent, where this construction is extensional in $\phi$ with respect to ${\sf PA}$-provable equivalence. We show that there can be no such extensional Rosser formula of any complexity. We give a positive answer to Hamkins' question for the case where we replace Extensionality by a weaker demand *Consistent Extensionality*. We also prove that we can demand the negation of $\rho$ to be $\Pi^0_1$-conservative, if we ask for the still weaker *Conditional Extensionality*. We show that an intensional version of the result for Conditional Extensionality cannot work.