I didn't know and I'm sure others are wondering too: PA probably means Peano Arithmetic. See https://en.m.wikipedia.org/wiki/Peano_axioms https://en.m.wikipedi

I didn't know and I'm sure others are wondering too: PA probably means Peano Arithmetic. See https://en.m.wikipedia.org/wiki/Peano_axioms https://en.m.wikipedia.org/wiki/Peano_axioms and https://math.stackexchange.com/a/213264 https://math.stackexchange.com/a/213264.