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Proof Techniques

Chapter 1  Proof techniques (section 2.1)

1.1  Theorems and Informal proofs

What we have seen so far:
P1/\···/\ Pn -> Q
how it's written
meaning in a given interpretation
Valid argument

True for all interpretations

True because of its very structure
Proofs of valid argument are purely based on syntactical rewriting rules.

Only arguments that are true for all interpretations can be proved.

1.1.1  Theorems

We are now interested to work in a particular subject (say integer arithmetic).
Définition 1   A theorem is an argument that is valid in this particular subject.
Exemple 1   If x is even and y is even then xy is even.

E(x): x is even

(for allx )
(for ally ) [E(x)/\ E(y) -> E(xy)]
This argument is true in this context, but not universally true.

How to prove it ?

Implicitly, we add new hypothesis which reflects basic facts of this subject.

For example, we add the following hypothesis: Using those new hypothesis, it's now possible to write a formal logic proof of the theorem.

See [, Example 4 p. 87]

1.1.2  Formal and informal proofs:

Remember that the goal of a proof is to be read by humans (in particular yourself) in order to convince those humans.

The formal proof above is convincing in the sense that you can check that all the steps are valid. However, it's very difficult to extract the meaning of the proof: The problem is that the keys of the proofs are buried under layers of insignificant details.

So, starting from now, we will write informal proofs:
Définition 2   An informal proof is a narrative descriptions, of the ideas and of the important steps of the proof, without extraneous details.
Exemple 2   A proof of the theorem above could be written as follow:

Proof. Let x and y be two even integers. We can take n and m such that x=2n and y=2m.

Then, xy=2(2nm). Since 2nm is an integer, xy is an even number.

Let see which details we omitted:
  1. Let x and y be two even integers.

    Implicit universal instantiation

  2. We can take n and m such that x=2n and y=2m.

    Implicit universal instantiation for x and y of the definition of an even number

    Implicit existential instantiation for n and m

  3. Then, xy=2(2nm)

    Implicit use of rules of arithmetic

  4. Since 2nm is an integer, xy is an even number:

    Implicit universal instantiation of the definition of an even number

    Implicit universal generalization to get the final result
Problem 1   Which details can we omit, and which not ?
A reader will be convinced by the proof if he can check that he could translate each step of the proof into one or several steps of a formal proof.

So, this all depends on WHO reads the proof!

You should not write your proof for your instructor, but for yourself, and for everybody else in the class.

A good rule of thumb is to imagine yourself rereading the proof in a few month, and to check that even then, you could possibly translate the proof into a formal one.

The difference between formal and informal proofs is very similar to the difference between assembly and, say, C++ or Java. A C++ program is not usable by itself. It's usable because it's possible to translate it into assembly language.

However, there does not exists, and most likely will never exists, a compiler that transforms informal proofs into formal proofs. English is much to rich a language for this.

1.1.3  Formal and informal theorems:

Most of the time, we also won't write theorems as a formal argument, but rather with an English sentence that could be translated into a formal argument:
Théorème 1   Let x and y be two integers. [Some definitions: interpretation]

Assume x and y are even. [Some hypothesis: P
1/\···/\ Pn]

Then, xy is even. [Consequent: Q]

1.1.4  To Prove or not to prove

In textbooks, you can be asked: prove that ... Usually, in real life, you first have to find what to prove. Two jobs:
Définition 3   A conjecture is a statement that you guess is true, but that you have not proved or disproved yet.
Classical steps:
  1. Explore some examples
  2. Try to see some pattern emerging
  3. Formulate a conjecture
  4. Try to prove (or disprove it)
Steps 1-3 are inductive reasoning, whereas step 4 is deductive reasoning.

Finding the good questions is as important as solving them !
Exemple 3   Fermat's conjecture.

1.1.5  Some ``research'' around the factorial

Définition 4   Let n be an integer. The factorial of n is the number n!:=n(n-1)···1.

For example, 1!=1 and 4!=4·3·2·1=24.
Problem 2   How big is n! ?

1.2  Proof techniques

We want to prove some argument of the form P -> Q.

1.2.1  Disproof by counter example

Conjecture 1   If n is a positive integer, then n!<n3.

1.2.2  Direct proof

Définition 5   Direct proof
  1. Assume P
  2. Deduce Q
Exemple 4   Prove that if x and y are even, then xy is even.
Exercice 1   Prove that if x and y are even, then x+y is even.

1.2.3  Proof by contraposition

Exemple 5   Prove that if n2 is odd, then n is odd.
Hint: prove instead that if n is even, then n2 is even.
Définition 6   Proof by contraposition:
  1. Assume Q' (the consequent is false)
  2. Prove P' (the antecedent is also false)
This technique relies on the fact that P -> Q is equivalent to Q' -> P'.
Exercice 2   Prove that xy is odd if and only if x and y are odd.

1.2.4  Exhaustive proof

Problem 1   Can a proof by example be valid ?

Yes, if there is a finite number of cases to be treated.
Exemple 6   Propositional logic formula (the truth table is finite)
Définition 7   Proof by exhaustion means that all cases have been exhausted.

(and so are you ...).
Exemple 7   Drawing a figure without lifting the pencil and without retracing a line.

1.2.5  Some ``research'' around rational numbers

Définition 8   A number x is rational if it can be written as p/q, where p and q are integers.
Exemple 8   5, -7/5, 1/-3, 14/4, 7/2, ... are rational.
Problem 2   Properties of rational numbers ?

Remarque 1   If x is rational, it's always possible to choose p and q so that:

Problem 3   Are all numbers rational ?

Problem 4   Assume x and y are rational.
  1. Is x+y rational ?
  2. Is xy rational ?
  3. Is x/y rational ?

Problem 5   Is the square root of an integer a rational number ?

Problem 6   Prove that the square root of 2 is irrational.

1.2.6  Proof by contradiction

Théorème 2   The square root of 2 is irrational.

Proof. Let's assume the square root of 2 is rational.

Let p and q be two integers such that (2)1/2=p/q and p and q are relatively prime.

Then, we have 2=(p/q)2, and so 2q2=p2.

Therefore p2 is even, and we have seen that this implies that p is also even.

Let k be the integer such that p=2k.

Then, we have 2q2=p2=(2k)2=4k2, and so q2=2k2.

It follows that q2 is even, and so q is also even.

Conclusion: p and q are both even.

That's a contradiction, since p and q are relatively prime!

Définition 9   Proof by contradiction
  1. Assume the contrary
  2. Deduce a contradiction
This technique relies on the fact that:
  1. Q/\ Q' is always false
  2. if P->0 is true, then P is false.

1.2.7  Serendipity

Exemple 9   The chess board problem.

1.3  Summary

Goal Technique Name
P -> Q Assume P ; deduce Q. Direct proof/Deduction method
P'-> Q' Prove Q-> P. Proof by contraposition
Q' Assume Q; deduce a contradiction. Proof by contradiction
P<-> Q Prove P-> Q; prove Q-> P.  
P/\ Q Prove P ; prove Q.  
P\/ Q Prove P'-> Q  
(P1\/ P2)-> Q Prove P1-> Q; prove P2-> Q Proof by cases
(for allx ) Q(x) Let x; prove Q(x) ui / ug
(there existsx ) Q(x) Construct a such that Q(a) eg
  Be smart Serendipity

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Proof Techniques / CSM MACS-358 / Nicolas M. Thiéry
Last modified: Fri Jan 19 13:12:17 2007