Argue by contradiction to show that there is no surjection...
14994
Showing a function $f$ cannot be surjective
113605
In Fitch, how does one prove "(P → Q)" from the premise "(¬P ∨ Q)"?
28452
Statement that is provable in $ZFC+CH$ yet unprovable in $ZFC+\lnot CH$
81428
Why does proof by contradiction not work on Rational numbers but only on Irrational numbers?
150041
T must be contravariantly valid
17073
Using proof by contradiction vs proof of the contrapositive
6535
Proof of for-all / there-exists "negation"?
35953
Prove that a function is not injective
53123
Trying to derive a contradiction with this simple inequality,
150707
Prove the statement 'World is not flat'
134120
Prove statement with a counter-example: For all sets S, T and V, V ∪ (S ∩ T) = (V ∪ S) ∩ T is false
113377
Empty preimage of an intersection implies empty intersection of the preimages
127733
How to prove De Morgan’s laws without using contradictions
106342
Why $L$-structures cannot satisfy a contradiction
33289
Negation of definition of continuity
147874
Why does the existence of independent statements not prove consistency?
27899
Can we prove the existence of $A\cup B$ without the union axiom?