Methods Of Mathematical Proof This Is From _A Random Walk In Science_ (by Joel E.

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Methods of Mathematical Proof

This is from _A Random Walk in Science_ (by Joel E. Cohen?):

To illustrate the various methods of proof we give an example of a
logical system.


Lemma 1. All horses are the same colour.
(Proof by induction)

Proof. It is obvious that one horse is the same colour. Let us assume
the proposition P(k) that k horses are the same colour and use this to
imply that k+1 horses are the same colour. Given the set of k+1 horses,
we remove one horse; then the remaining k horses are the same colour,
by hypothesis. We remove another horse and replace the first; the k
horses, by hypothesis, are again the same colour. We repeat this until
by exhaustion the k+1 sets of k horses have been shown to be the same
colour. It follows that since every horse is the same colour as every
other horse, P(k) entails P(k+1). But since we have shown P(1) to be
true, P is true for all succeeding values of k, that is, all horses are
the same colour.

Theorem 1. Every horse has an infinite number of legs.
(Proof by intimidation.)

Proof. Horses have an even number of legs. Behind they have two legs
and in front they have fore legs. This makes six legs, which is cer-
tainly an odd number of legs for a horse. But the only number that is
both odd and even is infinity. Therefore horses have an infinite num-
ber of legs. Now to show that this is general, suppose that somewhere
there is a horse with a finite number of legs. But that is a horse of
another colour, and by the lemma that does not exist.

Corollary 1. Everything is the same colour.

Proof. The proof of lemma 1 does not depend at all on the nature of the
object under consideration. The predicate of the antecedent of the uni-
versally-quantified conditional 'For all x, if x is a horse, then x is
the same colour,' namely 'is a horse' may be generalized to 'is anything'
without affecting the validity of the proof; hence, 'for all x, if x is
anything, x is the same colour.'

Corollary 2. Everything is white.

Proof. If a sentential formula in x is logically true, then any parti-
cular substitution instance of it is a true sentence. In particular
then: 'for all x, if x is an elephant, then x is the same colour' is
true. Now it is manifestly axiomatic that white elephants exist (for
proof by blatant assertion consult Mark Twain 'The Stolen White Ele-
phant'). Therefore all elephants are white. By corollary 1 everything
is white.

Theorem 2. Alexander the Great did not exist and he had an infinite
number of limbs.

Proof. We prove this theorem in two parts. First we note the obvious
fact that historians always tell the truth (for historians always take
a stand, and therefore they cannot lie). Hence we have the historically
true sentence, 'If Alexander the Great existed, then he rode a black
horse Bucephalus.' But we know by corollary 2 everything is white;
hence Alexander could not have ridden a black horse. Since the conse-
quent of the conditional is false, in order for the whole statement to
be true the antecedent must be false. Hence Alexander the Great did not
We have also the historically true statement that Alexander was warned
by an oracle that he would meet death if he crossed a certain river. He
had two legs; and 'forewarned is four-armed.' This gives him six limbs,
an even number, which is certainly an odd number of limbs for a man.
Now the only number which is even and odd is infinity; hence Alexander
had an infinite number of limbs. We have thus proved that Alexander the
Great did not exist and that he had an infinite number of limbs.