Theory of Computation Notes HCT 424, Lecture notes for Theory of Computation. University of Zimbabwe
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kudzai-shangwa21 April 2017

Theory of Computation Notes HCT 424, Lecture notes for Theory of Computation. University of Zimbabwe

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Introduction to theory of computation Course Objectives

1. To introduce the concepts and methods that underlie the formal (mathematical) study of computing machines

- What is a computing machine?

- How can we characterise and classify computing machines?

2. To present some of the basic results concerning the capabilities and limits of computing

machines

- Are there limits in principle to what can be computed?

• Every program computes a function from its input (a string of bits) to its output (a string of bits). Since a string of bits may be viewed as a binary number, every program may be viewed as computing a function from N to N. But is every function from N to N computable?

• Are all programming languages and computing machines equal (in principle)? Or are some more equal than others?

- Are there limits in practice to what can be computed?

• Are there computable problems which no matter

· how clever an algorithm we devise

· how efficient the language we write them in

· how ‘next generation’ the hardware

Will still not finish on inputs of small size before the heat death of the universe?

• How do we identify these problems?

3. To extend basic mathematical skills and to develop further logical and analytical skills

directly related to Computer Science.

4. To provide a theoretical foundation for other Computer Science courses

Definition of Theory of Computation Theory of computation is a branch of mathematics and computer science that deals with whether and how efficiently problems can be solved on a model of computation using an algorithm.

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The purpose of the Theory of Computation is to develop formal mathematical models of computation that reflect real-world computers.

Theory of computation is concerned with asking the following fundamental questions:

1. What are the limits of computation?

2. Are there problems which cannot be computed?

3. How do we model computation?

The first two questions can only be addressed after the last question is addressed, that is, how to represent computation in forms that admit rigorous analysis and not merely execution.

Branches of theory of computation Theory of computation is divided into three branches:

1. Complexity theory- In complexity theory, the objective is to classify problems as easy ones and hard ones. 2. Computability theory- in computability theory the classification of problems is by those that are solvable and those that are not. Computability theory introduces several of the concepts used in complexity theory. 3. Automata theory - deals with definitions and properties of different types of “computation models”. Examples of such models are:

Finite Automata. These are used in text processing, compilers, and hardware design.

Context-Free Grammars. These are used to define programming languages and in Artificial Intelligence.

Turing Machines. These form a simple abstract model of a “real” computer, such as our PC at home.

Central Question in Automata Theory: Do these models have the same power, or can one model solve more problems than the other?

Computer theory applications Theory of computation provides us with many good applications.

• Finite state machines are used in string searching algorithms, compiler design, control unit design in computer architecture, and many other modelling applications.

• Context free grammars and their restricted forms are the basis of compilers and parsing.

• NP-Complete theory helps us distinguish the tractable from the intractable. We do not focus on these applications. They are deep enough to require a separate course, and hopefully you have already seen some of them.

Relevance of theory to practice

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1. Theory gives us a new viewpoint of computers which are complex machines. Further theory shows us another elegant side of computation. This course can heighten our aesthetic sense and help us build more beautiful systems.

2. Theory provides conceptual tools that are used by practitioners in computer engineering. For example, when designing a new programming language for a specialised application you need an understanding of grammars. Further, an understanding of finite automata and regular expressions is useful with string searching and pattern matching.

3. This course helps you to learn problem solving skills. Theory teaches you how to think, prove, argue, solve problems, express yourself clearly and precisely, and abstract.

How do we model computation?

Computation is modelled using languages and machines.

Major areas of focus 1. Automata theory

2. Pushdown automata theory

3. Turing theory

Automata theory Is concerned with the definitions and properties of mathematical models of computation Unlike other models (SE, DB, DAA), computation models deal with all computers that exist, will exist and that can ever be dreamed of. Note that computational models may be accurate in some ways and not in other ways.

One model, called the finite automaton, is used in text processing, compilers, and hardware design.

Another model, called the context – free grammar, is used in programming languages and artificial intelligence.

Languages The fact that our study is sometimes called theory of formal languages makes it imperative to study languages. The word formal means that all the rules for the language are explicitly stated in terms of what strings of symbols can occur. The other reason why we study languages is that, languages are used to model computation. It has already been indicated that TOC deals with asking the question, how do we model computation? And it has been indicated that computation is modelled using languages and machines.

A language is defined as a game of symbols with formal rules. Natural languages like English are made up of letters, words, sentences, paragraphs etc. Similarly, with computer languages, certain character strings are recognisable as words (END, DO, WHILE etc), certain strings of words are

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recognisable as commands and certain sets of commands become a program that can be translated into machine commands.

Terminology 1. Alphabet (Г or Σ) (gamma and sigma) Is a finite set of fundamental units out of which we build structures (Cohen, 1991) or any finite set of symbols. Example Σ = {a, b, c, d... z} or Г = {0, 1}

2. Symbols Are members of an alphabet and usually denoted by small letters but numbers can be part of the symbols.

3. Words Are strings containing the symbols in some alphabet. Two words are considered the same if all their characters are the same and in the same order.

4. String over an alphabet- is a finite sequence of symbols from that alphabet written adjacent to one another and not separated by commas. For example if Σ = {a,b,c,d} then aadc, cdabb, and adcd are strings over Σ.

5. Empty string or null string

Is a string of length zero and is denoted by (Λ or ε). For clarity, the symbol Λ or ε is not allowed to be part of the alphabet for any language.

6. String length (|x| or length(x)) Refers to the number of symbols in a string. For any word x in any language, if length(x) = 0 then x = Λ

7. Language Is a certain specified set of strings of characters from an alphabet. Is denoted by L.

8. Emptylanguage (Φ) Is a language that has no words or strings.

Points of thought

is there a difference between Φ and Λ (language without words and word without symbols)

• is L + Λ = L (+ is the union of sets operation)

• is L + Φ = L Answers 1. There is a subtle but important difference between the word that has no letters and the language

that has no words. It is false that Λ is a word in the language Φ since this language has no words at all.

2. If a language L does not contain Λ, then L + Λ is not the same as L 3. L + Φ = L since no new words were added

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Cohen (1991) posits that, anyone who thinks that Λ is not confusing has missed something. It is already a problem and it gets worse latter.

Defining Languages There are two types of language defining rules:

1. Can be used to test whether a word is valid

2. Used to construct all the words in the language by some clear procedures

Concatenation operation Is used to join two or more strings and a concatenation is a string obtained by appending one string to the end of another. For example L1 = {good} and L2 = {one}, L1 + L2 = {goodone}

Reverse function If c is a word in some language L, then reverse(c) is the same string of letters spelled backward, called the reverse of c even if this backward string is not a word in L. Example reverse(eert) = tree

Palindrome Assume a new language Palindrome is defined over the alphabet, Σ = {m, n} then Palindrome = {Λ, and all strings y such that reverse(y) = y} so words in Palindrome are: {Λ, m, n, mm, nn, mnm, nmn, mmm...} Note that if you concatenate two words in Palindrome, the obtained word is sometimes in Palindrome.

Valid words If a word is contained in a given language it is valid otherwise it is invalid

Question: Given the following languages:

L1= pxqyrx+y, where x and y range over all the natural numbers, 0,1,2... and px denotes the string containing x successive copies of the symbol p

L2= pxqyrx-y, where x and y range over all the natural numbers, 0,1,2... and px denotes the string containing x successive copies of the symbol p and x > y For each language, list 5 valid and invalid words.

Kleene Closure of an alphabet Denoted by Σ*. This notation is sometimes known as the Kleene star.

Is defined as a language in which any string of letters from Σ is a word, even the null string. Example: if Σ = {b,c} then Σ* = { Λ, b, c bb, bc, cb, cc, bbb, bbc ... } The Kleene star is an operation that makes an infinite language of strings of letters out of an alphabet. The term infinite language means, infinitely many words each of a finite length.

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Lexicographic ordering Means that strings must be arranged in size order (words of shortest length first) and words of the same length must be put alphabetically. For example if Σ = {0, 1} then Σ* = {Λ, 0, 1, 00, 01, 10, 11, 000, 001, 010, 100, 110, 111 }

Closure of a set of words The use of the Kleene star can be generalised to sets of words not just sets of alphabet letters. If S is a set of words then S* is a set of all finite strings formed by concatenating words from S, where any word can be used as often as we like and where the null string is also included.

Example 1: if S = {00, 1} then S* = { Λ plus any word composed of factors of 00 and 1} = { Λ plus all strings of 0’s and 1’s in which 0’s occur in even clumps } = { Λ, 1, 00, 11, 001, 100, 111, 0000, 0011, 1001, 1100, 1111, 00001, 00100, 10000, 10011, 11001, 11100, 11111} The string 0010001 is invalid since it has a clump of 0’s of length 3.

Example 2: if S = {x, xy} then S* = { Λ plus any word composed of factors of x and xy} = { Λ plus all strings of x’s and y’s except those that start with y and those that contain a double y}

= { Λ, x, xx, xy, xxx, xxy, xyx, xxxx, xxxy, xxyx, xyxx, xyxy, xxxxx, xxyxx, xxyxy, xyxxx, xyxxy, xyxyx, ...}

Proving the existence of a word in the closure This is done by showing how a word can be written as a concatenate of words from the base set S. Using the last example, prove the existence of xxyxxxyx in S*.

Solution: factor the string as follows (x) (xy) (x) (x) (xy) (x). These six factors are all inset S so their concatenation is in S*. This factoring is unique sometimes it is not.

For example if S = {aa, aaa} then S* = { Λ plus all strings of more than one a} or {an for n = 0, 2, 3, 4, 5, ...} or { Λ, aa, aaa, aaaa, aaaaa, aaaaaa, ...} prove whether aaaaaaa is in S*. The factors are:

(aa) (aaa) (aa) or (aaa) (aa) (aa) or (aa) (aa) (aaa). Using example 1, prove whether or not the string 1000011001110001 is in the closure of S.

Proof by constructive algorithm Is a way of proving that something exists by showing how to create it.

Given that S= {aa, aaa}, prove that S* contains all an for n ≠1.

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We proceed as follows;

1. Assume that there are some powers of a we could not produce by concatenating factors of (aa) and (aaa). Since we can produce a4, a5and a6 then strings that we cannot produce must be large.

2. Determine the smallest power of a (> 1) that we cannot form out of factors of (aa) and (aaa). Assume here that we start making a list of how to construct the various powers of a. On this list we state how to form a,2 a 3 , a4, a5, a6 and so on. Assume that we work our way successfully up to an-1 but then we cannot figure out how to form an

3. Establish how an-2 was formed and then concatenate another factor of aa in front of this and then you will have an.

If Σ = {} then Σ* = {Λ} this is not the same as, if S = {Λ}, then S* ={Λ}which is also true but for a different reason that is Λ= Λ Λ.

Note: Λ is an element of L* for all languages. Sometimes the notation + instead of * is used to modify the concept of closure to refer to only the concatenation of some (not zero) strings from a set S. If Σ = {a} then Σ+ = {a, aa, aaa, aaaa, ...} For any language S* = S+ + Λ if S does not contain Λ. |Λ| = 0

Theorem 1 For any set S of strings, S* = S**

Illustration: if S = {xx, yyy} then S* is a set of strings where the x’s occur in even clumps and the y’s occur in groups of 3, 6, 9, ... some strings in S* are xxyyyxxxx yyy yyyxx. If we concatenate these three elements of S* we get one big word in S** which is also in S*. xxyyyxxxxyyyyyyxx = (xx) (yyy) (xx) (xx) (yyy) (yyy) (xx) This is analogous to saying that if computers are made up of circuits and circuits are made up of logic gates then computers are made up of logic gates. Proof Every word in S** is made up of factors from S*. Every factor from S* is made up of factors from S. Therefore, every word in S** is also a word in S*. This can be expressed as S** ⊂ S*. It can be generalised that for any set A we know that A⊂A*, since in A* we can choose as a word any one factor from A. So if we consider A to be our set S*, we have S* ⊂ S**. Together the two inclusions prove that S** = S*.

Ways of Defining Languages

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There are several ways of defining languages notably:

1. Recursive definition of languages

2. Regular expressions

3. Finite automata

4. Transition Graph

5. Other

Recursive definition

Is a method of defining sets and has three steps:

1. Specify some base objects in the set

2. Give rules for combining more objects in the set from the ones we already know

3. Declare that no objects except those constructed in this way are allowed in the set.

Examples:

1. Recursive definition of a set of positive even numbers

Rule 1: 2 is in EVEN

Rule 2: if x and y are both in EVEN then so is x+y

2. Recursive definition of positive integers

Rule 1: 1 is in INTEGERS

Rule 2: if x is in INTEGERS so is x+1

3. Recursive definition of integers

Rule 1: 1 is in INTEGERS

Rule 2: if both x and y are in INTEGERS, then so are x+y and x-y

4. Recursive definition of factorial

Rule 1: 0! = 1

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Rule 2: n!= n*(n-1)!

5. Recursive definition of polynomial

A polynomial is a finite set of terms, each of which is in the form: a real number times a power of x (that may be x0=1).

Rule 1: Any number is in POLYNOMIAL

Rule 2: the variable x is in POLYNOMIAL

Rule 3: if p and q are in POLYNOMIAL then so are p+q, p-q, pq and (p)

*Show that 2x2 + 3x – 10 is in POLYNOMIAL

By rule 1, 2 is in POLYNOMIAL

By rule 2, x is in POLYNOMIAL

By rule 3, (2)(x) is in POLYNOMIAL; call it 2x

By rule 3, (2x)(x) is in POLYNOMIAL; call it 2x2

By rule 1, 3 is in POLYNOMIAL

By rule 3, (3)(x) is in POLYNOMIAL

By rule 3, 2x2 + 3x is in POLYNOMIAL

By rule 1, -10 is in POLYNOMIAL

By rule 3, 2x2 + 3x + (-10) = 2x2 + 3x -10 is in POLYNOMIAL

REGULAR EXPRESSIONS

• Cohen (2001) defines REs as language defining symbols whereas Sipser (1996) defines them as expressions describing languages.

• Languages defined by REs are referred to as Regular Languages

• REs are limited in capacity because there are some languages that cannot be defined by REs • A RL is one that can be defined by a RE

• The value of a RE is a language.

Formal Definition of a Regular Expression

• Symbols that appear in REs include letters of the alphabet Σ, the symbol of the null string Λ, the symbol for the empty language Φ, parenthesis, the star operator and the plus sign.

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• The set of regular expressions is defined as follows:

• Rule 1: every letter of the alphabet Σ can be made into a regular expression by writing it in bold face; Λ itself is a RE and so is Φ.

• Rule 2: if r1 and r2 are REs then so are: ■ (r1)

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