Description: A version of aev 2055
with two universal quantifiers in the consequent.
One can prove similar statements with arbitrary numbers of universal
quantifiers in the consequent (the series begins with aeveq 2054, aev 2055,
aev2 2056).
Using aev 2055 and alrimiv 1925, one can actually prove (with no more axioms)
any scheme of the form (∀𝑥𝑥 = 𝑦 → PHI) , DV (𝑥, 𝑦) where
PHI involves only setvar variables and the connectors →, ↔,
∧, ∨, ⊤, =, ∀, ∃, ∃*, ∃!,
Ⅎ. An example is given by aevdemo 30489. This list cannot be
extended to ¬ or ⊥ since the scheme ∀𝑥𝑥 = 𝑦 is
consistent with ax-mp 5, ax-gen 1792, ax-1 6--
ax-13 2375 (as the
one-element universe shows), so for instance (∀𝑥𝑥 = 𝑦 → ⊥),
DV (𝑥, 𝑦) is not provable from these axioms
alone (indeed, dtru 5447
uses non-logical axioms as well). (Contributed by BJ,
23-Mar-2021.) |