Combinatorics.ppt

1
Combinatorics
Rosen 6th ed., §5.1-5.3, § 5.5

2
Combinatorics
• Count the number of ways to put things
together into various combinations.
e.g. If a password is 6-8 letters and/or digits,
how many passwords can there be?
• Two main rules:
– Sum rule
– Product rule

3
Sum Rule
• Let us consider two tasks:
– m is the number of ways to do task 1
– n is the number of ways to do task 2
– Tasks are independent of each other, i.e.,
• Performing task 1 does not accomplish task 2 and
vice versa.
• Sum rule: the number of ways that “either
task 1 or task 2 can be done, but not both”,
is m+n.
• Generalizes to multiple tasks ...

4
Example
• A student can choose a computer project from one of three
lists. The three lists contain 23, 15, and 19 possible
projects respectively. How many possible projects are there
to choose from?

5
Set Theoretic Version
• If A is the set of ways to do task 1, and B
the set of ways to do task 2, and if A and B
are disjoint, then:
“the ways to do either task 1 or 2 are
AB, and |AB|=|A|+|B|”

6
Product Rule
• Let us consider two tasks:
– m is the number of ways to do task 1
– n is the number of ways to do task 2
– Tasks are independent of each other, i.e.,
• Performing task 1does not accomplish task 2 and
vice versa.
• Product rule: the number of ways that “both
tasks 1 and 2 can be done” in mn.
• Generalizes to multiple tasks ...

7
Example
• The chairs of an auditorium are to be labeled with a letter
and a positive integer not to exceed 100. What is the
largest number of chairs that can be labeled differently?

8
Set Theoretic Version
• If A is the set of ways to do task 1, and B
the set of ways to do task 2, and if A and B
are disjoint, then:
• The ways to do both task 1 and 2 can be
represented as AB, and |AB|=|A|·|B|

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More Examples
• How many different bit strings are there of
length seven?

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More Examples
• Suppose that either a member of the CS faculty or a
student who is a CS major can be on a university
committee. How many different choices are there if there
are 37 CS faculty and 83 CS majors ?

11
More Examples
• How many different license plates are
available if each plate contains a sequence
of three letters followed by three digits?

12
More Examples
• What is the number of different subsets of a
finite set S ?

13
Example Using Both Rules
• Each user on a computer system has a password, which is
six to eight characters long where each character is an
uppercase letter or a digit. Each password must contain at
least one digit. How many possible passwords are there?

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IP Address Example
(Internet Protocol vers. 4)
• Main computer addresses are in one of 3 types:
– Class A: address contains a 7-bit “netid” ≠ 17, and a 24-bit “hostid”
– Class B: address has a 14-bit netid and a 16-bit hostid.
– Class C: address has 21-bit netid and an 8-bit hostid.
– Hostids that are all 0s or all 1s are not allowed.
• How many valid computer addresses are there?

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Example Using Both Rules:
IP address solution
• (# addrs)
= (# class A) + (# class B) + (# class C)
(by sum rule)
• # class A = (# valid netids)·(# valid hostids)
(by product rule)
• (# valid class A netids) = 27 − 1 = 127.
• (# valid class A hostids) = 224 − 2 = 16,777,214.
• Continuing in this fashion we find the answer is:
3,737,091,842 (3.7 billion IP addresses)

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Inclusion-Exclusion Principle
(relates to the “sum rule”)
• Suppose that km of the ways of doing task
1 also simultaneously accomplishes task 2.
(And thus are also ways of doing task 2.)
• Then the number of ways to accomplish
“Do either task 1 or task 2” is mnk.
• Set theory: If A and B are not disjoint, then
|AB|=|A||B||AB|.

17
Example
• How many strings of length eight either
start with a 1 bit or end with the two bit
string 00?

18
More Examples
• Hypothetical rules for passwords:
– Passwords must be 2 characters long.
– Each password must be a letter a-z, a digit 0-9,
or one of the 10 punctuation characters
!@#$%^&*().
– Each password must contain at least 1 digit or
punctuation character.

19
Sol. Cont’d
• A legal password has a digit or puctuation
character in position 1 or position 2.
– These cases overlap, so the principle applies.
• (# of passwords w. OK symbol in
position #1) = (10+10)·(10+10+26)
• (# w. OK sym. in pos. #2): also 20·46
• (# w. OK sym both places): 20·20
• Answer: 920+920−400 = 1,440

20
Pigeonhole Principle
• If k+1 objects are assigned to k places, then
at least 1 place must be assigned ≥2
objects.
• In terms of the assignment function:
If f:A→B and |A|≥|B|+1, then some element of B
has ≥2 pre-images under f.
i.e., f is not one-to-one.

21
Example
• How many students must be in class to guarantee that at
least two students receive the same score on the final
exam, if the exam is graded on a scale from 0 to 100
points?

22
Generalized Pigeonhole Principle
• If N≥k+1 objects are assigned to k places,
then at least one place must be assigned at
least N/k objects.
• e.g., there are N=280 students in this class.
There are k=52 weeks in the year.
– Therefore, there must be at least 1 week during
which at least 280/52= 5.38=6 students in
the class have a birthday.

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Proof of G.P.P.
• By contradiction. Suppose every place has
< N/k objects, thus ≤ N/k−1.
• Then the total number of objects is at most
• So, there are less than N objects, which
contradicts our assumption of N objects! □
N
k
N
k
k
N
k
k
N
k 







































1
1
1

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G.P.P. Example
• Given: There are 280 students in the class.
Without knowing anybody’s birthday, what
is the largest value of n for which we can
prove that at least n students must have
been born in the same month?
• Answer:
280/12 = 23.3 = 24

25
More Examples
• What is the minimum number of students required in a
discrete math class to be sure that at least six will receive
the same grade, if there are five possible grades, A, B, C,
D, and F?

26
Permutations
• A permutation of a set S of objects is an ordered
arrangement of the elements of S where each
element appears only once:
e.g., 1 2 3, 2 1 3, 3 1 2
• An ordered arrangement of r distinct elements of S
is called an r-permutation.
• The number of r-permutations of a set S with n=|S|
elements is
P(n,r) = n(n−1)…(n−r+1) = n!/(n−r)!

27
Example
• How many ways are there to select a third-
prize winner from 100 different people who
have entered a contest?

28
More Examples
• A terrorist has planted an armed nuclear bomb in
your city, and it is your job to disable it by cutting
wires to the trigger device.
• There are 10 wires to the device.
• If you cut exactly the right three wires, in exactly
the right order, you will disable the bomb,
otherwise it will explode!
• If the wires all look the same, what are your
chances of survival?
P(10,3) = 10·9·8 = 720,
so there is a 1 in 720 chance
that you’ll survive!

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More Examples
• How many permutations of the letters
ABCDEFG contain the string ABC?

30
Combinations
• The number of ways of choosing r elements
from S (order does not matter).
S={1,2,3}
e.g., 1 2 , 1 3, 2 3
• The number of r-combinations C(n,r) of a set
with n=|S| elements is
!
( , )
!( )!
n n
C n r
r r n r
 
 
 

 

31
Combinations vs Permutations
• Essentially unordered permutations …
• Note that C(n,r) = C(n, n−r)
)!
(
!
!
!
)!
/(
!
)
,
(
)
,
(
)
,
(
r
n
r
n
r
r
n
n
r
r
P
r
n
P
r
n
r
n
C














( , ) ( , ) ( , )
P n r C n r P r r


32
Combination Example
• How many distinct 7-card hands can be
drawn from a standard 52-card deck?
– The order of cards in a hand doesn’t matter.
• Answer C(52,7) = P(52,7)/P(7,7)
= 52·51·50·49·48·47·46 / 7·6·5·4·3·2·1
7
10 8
2
17
52·17·10·7·47·46 = 133,784,560

33
More Examples
• How many ways are there to select a committee to develop
a discrete mathematics course if the committee is to consist
of 3 faculty members from the Math department and 4
from the CS department, if there are 9 faculty members
from Math and 11 from CS?

34
Generalized
Permutations and Combinations
• How to solve counting problems where
elements may be used more than once?
• How to solve counting problems in which
some elements are not distinguishable?
• How to solve problems involving counting
the ways we to place distinguishable
elements in distinguishable boxes?

35
Permutations with Repetition
• The number of r-permutations of a set of n objects
with repetition allowed is
• Example: How many strings of length n can be
formed from the English alphabet?
r
n

36
Combinations with Repetition
• The number of r-combinations from a set with n
elements when repetition of elements is allowed
are C(n+r-1,r)

37
Example: How many ways are there to select 5 bills from a cash box
containing $1 bills, $2 bills, $5 bills, $10 bills, $20 bills, $50 bills, and
$100 bills? Assume that the order in which bills are chosen does not
matter and there are at least 5 bills of each type.

38
Approach: Place five markers in the compartments
i.e., # ways to arrange five stars and six bars ...
Solution: Select the positions of the 5 stars from 11 possible positions !
n=7
r=5
compartments
and
dividers markers
C(n+r-1,5)= C(7+5-1,5)=C(11,5)

39
• Example: How many ways are there to place 10
non-distinguishable balls into 8 distinguishable
bins?

40
Permutations and Combinations with
and without Repetition

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Permutations with
non-distinguishable objects
• The number of different permutations of n
objects, where there are non-distinguishable
objects of type 1, non-distinguishable objects
of type 2, …, and non-distinguishable objects
of type k, is
i.e., C(n, )C(n- , )…C(n- - -…- , )
1 2
!
! !... !
k
n
n n n
1
n
2
n
k
n
1
n 1
n 2
n 1
n 2
n 1
k
n  k
n
1 2 ... k
n n n n
   

42
Permutations with
non-distinguishable objects
• Example: How many different strings can be
made by reordering the letters of the word
SUCCESS

43
Distributing Distinguishable
Objects into Distinguishable Boxes
• The number of ways to distribute n
distinguishable objects into k distinguishable
boxes so that objects are placed into box i,
i=1,2,…,k, equals
i
n
1 2
!
! !... !
k
n
n n n

44
Distributing Distinguishable
Objects into Distinguishable Boxes
• Example: How many ways are there to distribute
hands of 5 cards to each of 4 players from the
standard deck of 52 cards?

Combinatorics.ppt

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