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Automata and Formal Languages - · PDF fileAutomata and Formal Languages ... Introduction, basic tools, ... • Formal languages – Regular languages – Context free languages. 6

Sep 06, 2018




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    Automata and Formal Languages

    Winter 2009-2010

    Yacov Hel-Or

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    What this course is all about

    This course is about mathematical modelsof computation

    Well study different machine models (finite automata, pushdown automata, Turing machine)

    and characterize what they can compute

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    Why should I care ?

    To understand the limits of computationsSome problems require more resources to compute

    and others cannot be computed at all.

    To learn some programming tools Automata show up in many different settings:

    compilers, text editors, hardware design, communication protocols, program proofing,

    To learn to think in a formal way about computing

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    Computability and Complexity

    Are there any problems that can not be solved by a (very powerful) computer?

    What makes some problems computationally hard and other easy?

    Can we partition the problems into classes such that problems in one class share the same computational properties?

    Complete answers: next semester.

    Introduction, basic tools, models, intuition: this semester.

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    This Course: Formal computational models

    The basic computational model : Finite State Automata

    Additional models: Pushdown automaton

    Turing machine

    Formal languages Regular languages

    Context free languages

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    Course Home page:

    Lecture notes will appear at the course web-page the day before the lecture (the latest).

    Proofs, examples, technical details will usually be presented on the board.

    No recitations, part of lecture time will be dedicated to solving problems.

    Grade calculation: Exam 70%, HW 30%

    Exam: Must pass the exam (60) in order to have the HW component.

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    Administration (cont.)

    HW: Will be given every week.

    Submission: Wednesdays before your lecture.

    Appeals: No more than two weeks after return.

    Grader + TA: Ilit Raz ([email protected])

    Newsgroup: news://

    Book: Sipser (see web-page for details).

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    Unit 1

    IntroductionMathematical Background

    Reading: Sipser, chapter 1

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    Set theory (review?)

    Logic, proofs (review?)

    Words and their operations:

    Languages and their operations:


    wwwww i ,,, *21 LLLLL i ,,, *21

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    Set TheoryA Set is a group of objects.The objects are called elements.


    1. By listing the elements. Examples: {3,5,7} ,{Alice, Bob, {1,2} } (for finite sets only)

    2. By providing a rule. Examples:

    {x | x is an odd integer between 2 and 8} ;

    {x | name of a student in the Automata class}

    A name of a set is usually a capital letter of theEnglish alphabet (A, B, C) or a capital letter withan index (X1 , X2 , X3)


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    The Empty Set

    A set with no elements is called an empty set.

    Notation or { }


    S - a set of all odd numbers that can be dividedby two without any remainder.

    S is an empty set.

    S = {xN | x is odd and x mod 2=0}=

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    Membership in a set

    s S , means that an element s is a memberof the set S

    b S, means that an element b is not a member of the set S

    Examples: 7 {21,7,30} and 8 {21,7,30}

    Let N be the set of natural numbers.2 N and 3.2 N

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    Given two sets A and B we say that A is asubset of B, if each element of A is also anelement of B.

    The notation: A B Formally: A B x A x B

    Example:A set of natural numbers N is a subset of a

    set of all real numbers R.N R

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    Proper Subset

    Given two sets A and B we say that A is aproper subset of B if each element of A isalso an element of B and there exists atleast one element in B that does not belongto A.

    The notation: A B

    Example:{1,3} {1, 2, 3, 8}

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    Some Facts

    Each set is a subset of itself:

    A A

    The empty set is a subset of every set:


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    More on Sets

    Two groups of elements that have the same elements but in different order form the same set.

    Example: If A = { 1,2,3,4}, B = { 2,1,4,3} then A = B

    Repetitions in a set are irrelevant

    Example: {1,2,3,4,2} = {1,2,3,4}

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    More on Sets

    The cardinality of a set is the number ofelements in the set. Notation: |A|

    Example: Let A = { 1,2,4,8,16}, then |A|=5

    A set can be:

    Finite A={Even integers smaller than 100}

    Infinite A={Even integers dividing 7 with no reminder}

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    Operations on sets union




    A visual model, called Venn diagram can be used.


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    Venn Diagram

    Start-t End-z Start-j

    terrifictheory topaz jazz

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    A binary operation

    The notation : A B


    A B = { x | xA or xB }

    A B

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    A binary operation

    The notation: A B


    A B = { x | xA and xB }

    A B

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    An unary operation

    The notation : ~A


    ~A = { x | x A}


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    Set Difference

    An binary operation

    The notation : A-B

    Formally: A-B = { x | xA and x B}

    Example: A = { 1,2,3 }, B={3,4,5}, A-B={1,2}

    A B

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    Power Set

    The power set is the set of all subsets of a given set.

    Notation: P(S) or 2S is a power set of S

    Note, that sets may appear as elements of other sets.

    The cardinality of a power set is: 2|S|


    Example:S = { 1,2 }P(S) = 2S = { ,{1},{2},{1,2}}

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    Sequences and Tuples

    A sequence of objects is a list of objects in some order.

    The notation : (7,21,57,)

    Unlike sets, the order and repetitions in the sequence do matter, thus

    (7,21,57) (7,57,21) and (7,21,57,57) (7,21,57)

    A sequence with k elements is a k-tuple

    Example: (7,21,57) is a 3-tuple.


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    Cartesian product

    A binary operation

    The Cartesian product (or cross product) is a set of all pairs (2-tuple) where the first element of the pair is in A and the second element of the pair is in B.

    The notation: AB

    A Cartesian product of a set with itself:

    AAA (k times)=Ak


    A = {2,3}, B = {b,c}, AB ={ (2,b), (2,c), (3,b), (3,c) }

  • Summary: sets

    ,{ } empty set

    a A membership

    |A| cardinality

    AB subset

    AB union

    AB intersection

    ~A complement

    A-B set difference

    2A power set

    (a,b,..) a sequence (k-tuple)

    AxB cartesian product 27

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    Letters and Alphabet

    Any finite set of letters (symbols) is called an alphabet.

    Notation: alphabet- ; Letter (symbol)-


    1 = { 0,1 } , 1=0 ; 2=1;

    2 = { a,b,c,d,e, }, 1=a ; 2=b ; 3=c ;

    3 = { 0,1,x,y,z}


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    Strings (and words)

    A string (word) over some alphabet is a finite sequence of letters from the alphabet.

    Example: = {0, 1}, w = 101

    The length of a word, w, denoted |w|, is the number of letters in it.


    w1 = abracadabra; |w1| = 11

    w2 = 001011 ; |w2| = 6

    , ,

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    More on Strings An empty word is a string without letters.

    The notation of an empty word is

    || = 0

    The number of occurrences of some letter in word w is denoted by #(W)


    Let w=aaba, then #a(w)=3, #b(w)=1, #c(w)=0

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    Operations on Strings






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    Reversing strings

    A reverse string is a string in which all letters are written in the opposite order

    Notation: wR

    Examples:w = 10, wR = 01

    s = abcb, sR = bcba

    A palindrome: a string w such that w=wR.

    Examples: aba, 010010,

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    A sub-string is a subsequence of consecutiveletters from a string

    Example: Let w = 101. All sub-strings of w are:

    B(w) = { , 1, 0, 10, 01, 101 }.

    Note: 11 B(w)

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    Prefixes A prefix is a sub-string which starts from

    the first letter of the word (or an empty word).

    A proper prefix of a string is a prefix that is not equal to the complete word.

    Example: Let w = acdb. The prefixes of w are { , a,ac,acd,acdb}. acdb is not a proper prefix.


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    Suffixes A suffix is a substring ending with the last

    letter of the word or an empty word.

    A proper suffix of the string is a suffix that is not the whole word.

    Example: Let w = acdb. The suffixes of w are { ,b, db,cdb,acdb}. acdb is not a proper suffix

    Note: The prefixes of w are the reversed suffixes of wR.


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    A binary operation (over two words)

    The concatenation of two words x and yplaces them one after the other such thatthe first word is a prefix and a second oneis a suffix.

    Notation: concatenation of two words x andy: xy

    Example: Let x= 01, y=012, z=10

    Then xy=01012, yx=01201, xyz=0101210

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    Concatenation (Cont.)

    The result of concatenating a word with anempty word is the string itself. Forexample

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