$\neg (\neg A) \rightarrow A$ part of the axioms of propositional logic?
138897
help with some Hilbert style proofs in a propositional logic axiom system.
47161
Quick logic question about $P\leftrightarrow Q$, terminology
93327
Negation of if and only if?
151356
Need Hints Prove "$((\neg \alpha \to \alpha) \to \alpha) $" Using Axiom 1,2,3 and MP and deduction theorem
65308
Axiom of Choice: Family of non-empty sets mutually disjoint or not?
63963
deductive system where the axioms are all the propositions that are not tautology
9282
Prove using natural deduction $((P \land Q) \rightarrow R) \vdash (P \rightarrow R) \lor (Q\rightarrow R)$
6535
Proof of for-all / there-exists "negation"?
65379
Deriving $\neg R$ from $\{R↔(R∨(P∧¬P)), R↔¬P, ¬P→(P↔(Q→Q)), P→Q\}$
72034
Truth table and the meaning of $\oplus$ in propositional logic
134384
What does $\rightarrow$ mean in $p \rightarrow q$
87955
How to prove that $P \rightarrow Q$ is equivalent with $\neg P \lor Q $?
84251
Show that $\Gamma \cup \{\neg \phi\}$ is satisfiable if and only if $\Gamma\not \models \phi$
45323
How to prove $\vdash\neg P\to (P\to Q)$?
101741
Selection of logical connectives {¬,∨,∧,⇒,⇔} in set theory?
77297
Counterexample for $A,B\triangleleft G,G/A\cong B \Rightarrow G/B\cong A$
140194
Using natural deduction to prove that $p \implies q \vdash \lnot p \lor q$