How to prove that ├ (¬¬φ⇒φ) and ├ (φ⇒¬¬φ) based on deduction and/or proof by contradiction?
32929
Questions about proving $\lnot \lnot a = a$
52322
Implication in mathematics - How can A imply B when A is False?
43700
Are proofs by contradiction really logical?
48213
proving logical equivalence $(P \leftrightarrow Q) \equiv (P \wedge Q) \vee (\neg P \wedge \neg Q)$
72185
The negation of a limit condition, Spivak style.
46939
Natural deduction, Proof $\vdash$ $P\Rightarrow(Q\Rightarrow P)$
43462
Disprove $\mathcal{P}(A\Delta B) \setminus \{\emptyset\} = \mathcal{P}(A) \Delta\mathcal{P}(B) $. Is my proof good?
28452
Statement that is provable in $ZFC+CH$ yet unprovable in $ZFC+\lnot CH$
32441
How to prove or disprove statements about sets
17073
Using proof by contradiction vs proof of the contrapositive
140194
Using natural deduction to prove that $p \implies q \vdash \lnot p \lor q$
77759
How do i prove that $(p \vee q) \wedge (\neg p \vee r) \rightarrow (q \vee r)$ is a tautology without using the truth table?
79407
How to solve ~(P → Q) : P & ~Q by natural deduction?
85770
What is the difference between ∀x(p(x) →q(x)) and ∀x(p(x) ∧q(x))?
100399
A doubt with a part of a certain proof.
134120
Prove statement with a counter-example: For all sets S, T and V, V ∪ (S ∩ T) = (V ∪ S) ∩ T is false
88104
Prove "$(p \lor q) \land (\neg p \lor r)) \rightarrow (q \lor r)$" is a tautology