Wikipedia:Reference desk/Archives/Mathematics/2016 August 11
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August 11
editPeano Arithmetic: Are there propositions provable in PA, yet not in the following similar system?
editI'm asking, because I'd like to replace the (first order) axiom of induction by a new axiom claiming that every positive number can - be subtracted from every bigger (or not smaller) number - and divide some number lying in the "immediate local neighborhood" of the bigger number, i.e. , and I hope this new axiom is not weaker than the (first order) axiom of induction. HOTmag (talk) 09:43, 11 August 2016 (UTC)
- Yes. First order PA is not finitely axiomatizable, which is why the axiom of induction is actually an axiom schema, necessarily.
- I don't have a reference handy other than Simpson's book, but for every n there is a model of (Robinson Arithmetic) + ( -induction) + (not -induction). Recall that PA is (Robinson Arithmetic) + ( -induction for all n). So consider A a putative finite axiomatization of PA. Since proofs are finite, A is provable from a finite fragment of the standard axiomatization of PA, which means that there's some n such that A is provable from (Robinson Arithmetic) + ( -induction). So there is a structure that models A but not PA, and thus A is not an axiomatization of PA.
- So there is some n such that your axiom cannot prove -induction. I suspect n is 1 or 2.--2406:E006:178C:1:781F:A651:32E2:B81E (talk) 10:20, 11 August 2016 (UTC)
- I still wonder how such a proposition (unprovable in my axiom system) looks like. HOTmag (talk) 10:39, 11 August 2016 (UTC)
- Okay. Your axiom can be proved with bounded induction, which is weaker than -induction. It's known that -induction can prove that Ackermann's function is total, but -induction cannot. So this is a statement unprovable in your axiom system.--2406:E006:178C:1:781F:A651:32E2:B81E (talk) 10:51, 11 August 2016 (UTC)
- All right. Now I wonder if there are propositions, provable in -induction, yet not in the system I've suggested. HOTmag (talk) 11:16, 11 August 2016 (UTC)
- Okay. Your axiom can be proved with bounded induction, which is weaker than -induction. It's known that -induction can prove that Ackermann's function is total, but -induction cannot. So this is a statement unprovable in your axiom system.--2406:E006:178C:1:781F:A651:32E2:B81E (talk) 10:51, 11 August 2016 (UTC)
- I still wonder how such a proposition (unprovable in my axiom system) looks like. HOTmag (talk) 10:39, 11 August 2016 (UTC)