STPL := soundness theorem for predicate logic
(see this)
When trying to figure out the strength of the STPL in reverse mathematics, I managed to convince myself of the following:
a) ACA0 has a (provably) $\Delta_1^1$ pair of formulas, which it proves enough about to consider them as defining in it the truth predicate for first-order structures.
b) ACA0 does not prove the STPL using the truth predicate as defined in (a).
c) [ACA0 + [$\Delta_1^1$ induction]] does prove the STPL as given in (b).
(EDIT: Based on François's answer, I now believe that I was wrong about (a). First, the two formulas I was thinking of aren't provably equivalent; second, the $\Sigma_1^1$ formula, which iscomes closer to being correctworking, does not provably satisfy ($\operatorname{True}(\lnot p) \leftrightarrow \lnot \operatorname{True}(p))$).
So, my questions are:
- Are my understandings correct?
- Does ACA0 + STPL prove $\Delta_1^1$ induction?
- Is anything else known about the positions of STPL and $\Delta_1^1$ induction in the reverse mathematics hierarchy? (For example, where would they go on the list on page 4 here?)