Re: Provable in T?

Aatu Koskensilta wrote:
The theory ISigma_1 is just Robinson arithmetic + induction for Sigma_1
formulas.

Would you specify what a Sigma_1 formula is?

ISigma_1 is stronger than PRA and Robinson arithmetic.

What does the 'I' stand for in 'ISigma_1'?

Is ISigma_1 in some sense the least theory to which the incompleteness
theorem applies? Since it's stronger than PRA and Robinson arithmetic,
should I take it that incompleteness does not apply to them? And should
I take it that there's no common theory that is between Robinson
arithmetic and ISigma_1 in terms of strength?

MoeBlee

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