Languages and Finite Automata

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Introduction to Theoretical Computer
Science COMP 335 Fall 2004 Slides by Costas
Busch, Rensselaer Polytechnic Institute, Modified
by N. Shiri G. Grahne, Concordia University
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• This course A study of
• abstract models of computers and computation.
• Why theory, when computer field is so practical?
• Theory provides concepts and principles, for both
  hardware and software that help us understand the
  general nature of the field.

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Mathematical Preliminaries
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• Mathematical Preliminaries
• Sets
• Functions
• Relations
• Graphs
• Proof Techniques

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SETS

A set is a collection of elements
We write
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Set Representations C a, b, c, d, e, f, g,
h, i, j, k C a, b, , k S 2, 4, 6,
S j j gt 0, and j 2k for kgt0 S
j j is nonnegative and even
finite set
infinite set
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A 1, 2, 3, 4, 5
Universal Set all possible elements
U
1 , , 10
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• Set Operations
• A 1, 2, 3 B 2, 3, 4, 5
• Union
• A U B 1, 2, 3, 4, 5
• Intersection
• A B 2, 3
• Difference
• A - B 1
• B - A 4, 5

2
4
1
3
5
U
2
3
1
Venn diagrams
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• Complement
• Universal set 1, , 7
• A 1, 2, 3 A 4, 5, 6, 7

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A
A
6
3
1
2
5
7
A A
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even integers odd integers
Integers
1
odd
0
5
even
6
2
4
3
7
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DeMorgans Laws
A U B A B
U
A B A U B
U
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Empty, Null Set

S U S S S - S
- S
U
Universal Set
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Subset
A 1, 2, 3 B 1, 2, 3, 4,
5
Proper Subset
B
A
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Disjoint Sets
A 1, 2, 3 B 5, 6
A
B
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Set Cardinality

• For finite sets

A 2, 5, 7 A 3
(set size)
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Powersets
A powerset is a set of sets
S a, b, c
Powerset of S the set of all the subsets of S
2S , a, b, c, a, b, a, c, b,
c, a, b, c
Observation 2S 2S ( 8 23 )
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Cartesian Product
A 2, 4 B 2, 3, 5 A
X B (2, 2), (2, 3), (2, 5), ( 4, 2), (4, 3),
(4, 5) A X B AB Generalizes to more
than two sets A X B X X Z
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FUNCTIONS
domain
range
B
A
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f(1) a
a
1
2
b
c
3
5
f A -gt B
If A domain then f is a total function
otherwise f is a partial function
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RELATIONS
R (x1, y1), (x2, y2), (x3, y3),
xi R yi e. g. if R gt 2 gt 1,
3 gt 2, 3 gt 1
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Equivalence Relations

• Reflexive x R x
• Symmetric x R y y R x
• Transitive x R y and y R z
  x R z
• Example R
• x x
• x y y x
• x y and y z x z

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Equivalence Classes
For an equivalence relation R, we define
equivalence class of x xR y x R
y Example R (1, 1),
(2, 2), (1, 2), (2, 1), (3, 3),
(4, 4), (3, 4), (4, 3) Equivalence class of
1R 1, 2 Equivalence class of 3R 3, 4
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GRAPHS
A directed graph GltV, Egt
e
b
node
d
a
edge
c

• Nodes (Vertices)
• V a, b, c, d, e
• Edges
• E (a,b), (b,c), (b,e),(c,a), (c,e),
  (d,c), (e,b), (e,d)

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Labeled Graph
2
6
e
2
b
1
3
d
a
6
5
c
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Walk
Walk is a sequence of adjacent edges
(e, d), (d, c), (c, a)
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Path
A path is a walk where no edge is repeated A
simple path is a path where no node is repeated
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Cycle
e
base
b
3
1
d
a
2
c
A cycle is a walk from a node (base) to itself A
simple cycle only the base node is repeated
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Trees
root
parent
leaf
child
Trees have no cycles Ordered trees?
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root
Level 0
Level 1
Height 3
leaf
Level 2
Level 3
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PROOF TECHNIQUES

• Proof by induction
• Proof by contradiction

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Induction
We have statements P1, P2, P3,

• If we know
• for some b that P1, P2, , Pb are true
• for any k gt b that
• P1, P2, , Pk imply Pk1
• Then
• Every Pi is true, that is, ?i P(i)

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Proof by Contradiction

• We want to prove that a statement P is true
• we assume that P is false
• then we arrive at an incorrect conclusion
• therefore, statement P must be true

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Example
Theorem is not rational Proof Ass
ume by contradiction that it is rational
n/m n and m have no common
factors We will show that this is impossible
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n/m 2 m2 n2
n is even n 2 k
Therefore, n2 is even
m is even m 2 p
2 m2 4k2
m2 2k2
Thus, m and n have common factor 2
Contradiction!
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Pigeon Hole Principle If n1 objects are put
into n boxes, then at least one box must contain
2 or more objects. Ex Can show if 5 points are
placed inside a square whose sides are 2 cm long
? at least one pair of points are at a distance
?2 cm. According to the PHP, if we divide the
square into 4, at least two of the points must be
in one of these 4 squares. But the length of the
diagonals of these squares is ?2. ? the two
points cannot be further apart than ?2 cm.
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Languages
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• A language is a set of strings
• String A sequence of letters/symbols
• Examples cat, dog, house,
• Defined over an alphabet

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Alphabets and Strings

• We will use small alphabets
• Strings

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String Operations
Concatenation
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Reverse
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String Length

• Length
• Examples

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Length of Concatenation

• Example

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The Empty String

• A string with no letters
• Observations

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Substring

• Substring of string
• a subsequence of consecutive characters
• String
  Substring

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Prefix and Suffix

• Prefixes Suffixes

prefix
suffix
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Another Operation

• Example
• Definition

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The Operation

• the set of all possible strings from
• alphabet

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The Operation
the set of all possible strings from
alphabet except
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Languages

• A language is any subset of
• Example
• Languages

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Note that
Sets
Set size
Set size
String length
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Another Example

• An infinite language

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Operations on Languages

• The usual set operations
• Complement

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Reverse

• Definition
• Examples

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Concatenation

• Definition
• Example

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Another Operation

• Definition
• Special case

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More Examples

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Star-Closure (Kleene )

• Definition
• Example

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Positive Closure

• Definition