Proofs By ContradictionThis is also called reductio ad absurdum. Reductio ad absurdum, which Euclid loved so much, is one of a mathematician's finest weapons. It is a far finer gambit than any chess gambit: a chess player may offer the sacrifice of a pawn or even a piece, but a mathematician offers the game G.H. Hardy, A Mathematician's Apology, 1940. To prove a theorem Consider a simple example. Theorem For every Proof We will prove by contradiction. The original statement is The negation of this statement is Proof by Contradiction (Example)
Let us assume that the original statement is false. It's negation must be true for some
Assuming the negation of the theorem leads to a contradiction. Therefore, the theorem is true. (QED) Proof By ContradictionLet us look at how such proofs look like. Universal StatementTo prove: Proof
Assume, for the sake of contradiction, that the statement does not hold. In other words, there is a number Universal ImplicationThis is a special case of universal statements. To prove: Proof
Assume, for the sake of contradiction, that the statement does not
hold. Therefore, there is a number Existential StatementTo prove Examples of Proof By ContradictionHere are some famous reductios. Theorem There are infinitely many prime numbers. Proof
Proof is by contradiction. Let us assume that there are finitely many (let us say
Here is another one from Euclid. Theorem The number Proof
Proof is again by contradiction. Let us assume that
We know that
This is a contradiction since |