Mathematics for Computer Science
MIT 6.042J/18.062J
Proof by Contradiction
Proof by Cases
Albert R Meyer
September 9, 2011
lec 1F.1
Proof by Contradiction
If an assertion implies
something false, then
the assertion itself
must be false!
Albert R Meyer
September 9, 2011
lec 1F.2
Proof by Contradiction
1331
If so, 1332 
£ 11
3
That’s not true, so
3
Albert R Meyer
1332  11
September 9, 2011
lec 1F.3
Proof by Contradiction
Theorem:
2 is irrational.
• Suppose 2 was rational
• So have n, d integers without common
prime factors such that
n
2=
d
• We will show that n & d are both even.
This contradicts no common factor.
Albert R Meyer
September 9, 2011
lec 1F.4
Proof by Contradiction
Theorem:
n
2=
d
2d =n
2
2
2d = n
2 is irrational.
so can assume
QED
So n is even
Albert R Meyer
n = 2k
2
2
n = 4k
2d = 4k
2
2
d = 2k
2
2
So d is even
September 9, 2011
lec 1F.5
Quickie
Proof assumes that
2
if n is even, then n is even.
Why is this true?
Albert R Meyer
September 9, 2011
lec 1F.6
Mathematics for Computer Science
MIT 6.042J/18.062J
Proof by Cases
Albert R Meyer
September 9, 2011
lec 1F.7
Java Logical Expression
if ((x>0) || (x <= 0 && y>100))
OR
AND
(more code)
better: if ((x>0) || y>100)
(more code)
Albert R Meyer
September 9, 2011
lec 1F.8
Case 1: x > 0
true
if ((x>0) || (x <= 0 && y>100))
AND
OR
true
if ((x>0) || y>100)
OR
so both are true
Albert R Meyer
September 9, 2011
lec 1F.9
Case 2: x 0
false
if ((x>0) || (x <= 0 && y>100))
AND
OR
false
if ((x>0) || y>100)
OR
Albert R Meyer
September 9, 2011
lec 1F.10
Case 2: x 0
true
if
(x <= 0 && y>100)
AND
(
if ((x>0)
|| y>100)
Albert R Meyer
September 9, 2011
lec 1F.11
Case 2: x 0
if
(
y>100)
(
if ((x>0)
|| y>100)
so both still the same
Albert R Meyer
September 9, 2011
lec 1F.12
Proof by Cases
Reasoning by cases can break a
complicated problem into
easier subproblems.
Some philosophers* think
reasoning this way is worrisome.
*intuitionists
Albert R Meyer
September 9, 2011
lec 1F.13
$1,000,000 Question
Is P = NP ?
Albert R Meyer
September 9, 2011
lec 2M.28
$1,000,000 Question
The answer is on my
desk!
(Proof by Cases)
Albert R Meyer
September 9, 2011
lec 1F.29
Team Problems
Problems
1―4
Albert R Meyer
September 9, 2011
lec 1F.30