Proving a sequent is valid without a premise
70117
Show that $(p \to q) \lor (q \to p)$ is a tautology
14849
Decidability of a predicate.
137494
Why is "If ψ ∈ Γ then the sequent (Γ ⊢ ψ) is correct" true?
57215
Stuck on formal proofs. Not sure how to continue
31299
Using the Fitch system, how do I prove $((p \implies q) \implies p)\implies p$?
78290
Countable axiom of choice: why you can't prove it from just ZF
65895
Irrationality proofs not by contradiction
15354
Formal proof of $\exists x \forall y P(x,y) \implies \forall y \exists x P(x,y)$
143627
Use the Fitch system to prove the tautology (p ∨ ¬p)
9077
Proof that no term (of a language of predicate logic) is a (non-empty) initial segment of another
22193
Why, logically, is proof by contradiction valid?
46939
Natural deduction, Proof $\vdash$ $P\Rightarrow(Q\Rightarrow P)$
121361
Showing that $\lnot Q \lor (\lnot Q \land R) = \lnot Q$ without a truth table
43700
Are proofs by contradiction really logical?
126870
Show that axiom of replacement implies from the Axiom of Specification [Proof Verification]
76347
Shorter proof for some equvalences
63963
deductive system where the axioms are all the propositions that are not tautology