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Proof Methods ,
, ~, , 
•
Purpose of Section:Most theorems in mathematics take the
form of an implication P  Q or as a biconditional
P  Q , where the biconditional can be verified by proving both
P Q and Q P . We will study a variety of ways of proving
PQ including a direct proof, proof by contrapositive, and
three variations of proof by contradiction, including proof by
reductio ad absurdum
Instructor: Hayk Melikya
Introduction to Abstract Mathematics
[email protected]
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Many theorems in mathematics have the form of an
implication PQ, where one assumes the validity of P , then
with the aid of existing mathematical facts and theorems as
well as laws of logic and reasoning, arrive at the conclusion Q.
Although the goal is always to “go from P to Q,” is more than
one valid way of achieving this goal.
Five equivalent ways to proving the implication PQ are
shown in next
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Five forms
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Example 1
(Direct Proof) If n is an odd natural number, then n2 is odd.
Proof: An integer n is called odd if it is of the form n = 2k +1
for some integer k , then since we assumed n odd, we can
write n = 2k +1. Squaring gives
n2 = (2k +1)2
= 4k2 + 4k + 1
= 2(2k2 + 2k ) + 1
Since k is a natural number we know s = 2k2 + 2k
is a natural number and so n2 = 2s +1 which proves that n2 is
odd.
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You should remember the symbols used to denote for each set such as
increasing collection of set N, Z, Q, R, C since we will be referring
to these sets in the remainder of the book.
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Direct Proof [PQ]
A direct proof starts with the given assumption P and uses existing facts to
establish the truth of the conclusion Q. More formally:
Let H1, H2, … Hk , P and Q be a propositional expressions then
H1, H2, … Hk ├ P  Q
if and only if
H1, H2, … Hk, P ├ Q .
The Rule of Direct Proof [DP] (often called Deduction Theorem) can be
shown to be true by use of Exportation (see the theorem ):
H1, H2, … Hk,  (P  Q)  (H1  H2  …  Hk  P )  Q .
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Example: Prove (P  Q)  (R  S)├ P  R.
Proof:
1. (P  Q)  (Q  R)
(Goal P  R)
2. P
3. P  Q
4. Q  R
5. R
6. P  R.
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Hyp
Assum
2 Add
2, 1 MP
4S
25 DP
END
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We can use this approach to write paragraphs proofs also.
Always follow the following steps while writing paragraph proofs:
5.
Identify hypothesis and conclusion of implication,
assume the hypothesis
translate the hypothesis (replace more useable equivalent form),
write comment regarding what must be shown,
translate the comment (replace more useable equivalent form)
6.
deduce the conclusion
1.
2.
3.
4.
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Example: Let x be an integer. Prove that if x is an odd integer
then x + 1 is even integer.
Let P:=” x is odd integer”
(hypothesis)
and
Q := “x + 1 is even integer ”
(conclusion).
What we want
├PQ
Assume P:
Translate: by definition x is even if x = 2k +1 for some integer k
Then
x + 1 = (2k + 1) +1
= 2k + 2
= 2(k + 1)
Indirect proofs2 refer to proof by contrapositive or proof by contradiction
which we introduce here
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Indirect Proofs:
Indirect proofs refer to proof by contrapositive or proof by
contradiction which we introduce next .
A contrapositive proof or proof by contrapositive for
conditional proposition P  Q one makes use of the tautology
(P Q)  (  Q   P).
Since P  Q and  Q   P are logically equivalent we first
give a direct proof of  Q   P and then conclude that P  Q.
One type of theorem where proof by contrapositive is often used is where
the conclusion Q states something does not exist.
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Here is the structure of contrapositive proof of P  Q
Proof:
Assume,  Q,
.
.
.
Therefore,  P.
Thus,  Q   P
Therefore, P  Q.
END
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Contradictions -1:
Let H1, H2, … Hk and Q be a propositional expressions then
H1, H2, … Hk ├ Q
if and only if
H1 , H2 , … Hk  Q ├ P   P
for some propositional expression P.
This is also known as proof by contradiction or
reductio ad absurdum.
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Contradictions-2 [ ( P   Q)   P or (P   Q)  Q ]
A proof by contradiction is based on the tautology
P  (P  (Q Q)).
Here the hypotheses P to be true but the conclusion Q false,
and from this reach some type of contradiction, either
contradicting the assumption P or contradicting the denial  Q
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Reductio ad absurdum [ ( P   Q)  R   R ].
Reductio ad absurdum is another form of proof by
contradiction.
After assuming P is true and Q is not true, one seeks to prove
an absurd result (like 1 = 0 or x2 < 0 ),
which we denote by R   R .
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Not Mathematical Proofs:
▶ The proof is so easy we’ll skip it.
▶ Don’t be stupid, of course it’s true!
▶ It’s true because I said it’s true!
▶ Oh God let it be true!
▶ I have this gut feeling.
▶ I did it last night.
▶ I define it to be true!
You can think of a few yourself.
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Examples:
Example 6 (Proof by Contrapositive) :
Let n be a natural number. If n2 is odd so is n.
Example 7 (Proof by Contradiction):
2 is irrational number.
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Statement forms in Mathematics
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