Module 1: Course Overview

1
Module 1 Course Overview

• Course CSE 460
• Instructor Dr. Eric Torng
• Grader/TA To be determined

2
What is this course?

• Philosophy of computing course
• We take a step back to think about computing in
  broader terms
• Science of computing course
• We study fundamental ideas/results that shape the
  field of computer science
• Applied computing course
• We learn study a broad range of material with
  relevance to computing today

3
Philosophy

• Phil. of life
• What is the purpose of life?
• What are we capable of accomplishing in life?
• Are there limits to what we can do in life?
• Why do we drive on parkways and park on
  driveways?

• Phil. of computing
• What is the purpose of programming?
• What can we achieve through programming?
• Are there limits to what we can do with programs?
• Why dont debuggers actually debug programs?

4
Science

• Physics
• Study of fundamental physical laws and phenomenon
  like gravity and electricity
• Engineering
• Governed by physical laws

• Our material
• Study of fundamental computational laws and
  phenomenon like undecidability and universal
  computers
• Programming
• Governed by computational laws

5
Applied computing

• Applications are not immediately obvious
• In some cases, seeing the applicability of this
  material requires advanced abstraction skills
• Every year, there are people who leave this
  course unable to see the applicability of the
  material
• Others require more material in order to
  completely understand their application
• for example, to understand how regular
  expressions and context-free grammars are applied
  to the design of compilers, you need to take a
  compilers course

6
Some applications

• Important programming languages
• regular expressions (perl)
• finite state automata (used in hardware design)
• context-free grammars
• Proofs of program correctness
• Subroutines
• Using them to prove problems are unsolvable
• String searching/Pattern matching
• Algorithm design concepts such as recursion

7
Fundamental Theme

• What are the capabilities and limitations of
  computers and computer programs?
• What can we do with computers/programs?
• Are there things we cannot do with
  computers/programs?

8
Module 2 Fundamental Concepts

• Problems
• Programs
• Programming languages

9
Problems

• We view solving problems as the main application
  for computer programs

10
Definition

• A problem is a mapping or function between a set
  of inputs and a set of outputs
• Example Problem Sorting

(4,2,3,1)
(1,2,3,4)
(3,1,2,4)
(1,5,7)
(7,5,1)
(1,2,3)
(1,2,3)
11
How to specify a problem

• Input
• Describe what an input instance looks like
• Output
• Describe what task should be performed on the
  input
• In particular, describe what output should be
  produced

12
Example Problem Specifications

• Sorting problem
• Input
• Integers n1, n2, ..., nk
• Output
• n1, n2, ..., nk in nondecreasing order
• Find element problem
• Input
• Integers n1, n2, , nk
• Search key S
• Output
• yes if S is in n1, n2, , nk, no otherwise

13
Programs

• Programs solve problems

14
Purpose

• Why do we write programs?
• One answer
• To solve problems
• What does it mean to solve a problem?
• Informal answer For every legal input, a correct
  output is produced.
• Formal answer To be given later

15
Programming Language

• Definition
• A programming language defines what constitutes a
  legal program
• Example a pseudocode program may not be a legal
  C program which may not be a legal C program
• A programming language is typically referred to
  as a computational model in a course like this.

16
C

• Our programming language will be C with minor
  modifications
• Main procedure will use input parameters in a
  fashion similar to other procedures
• no argc/argv
• Output will be returned
• type specified by main function type

17
Maximum Element Problem

• Input
• integer n 1
• List of n integers
• Output
• The largest of the n integers

18
C Program which solves the Maximum Element
Problem

• int main(int A, int n)
• int i, max
• if (n lt 1)
• return (Illegal Input)
• max A0
• for (i 1 i lt n i)
• if (Ai gt max)
• max Ai
• return (max)

19
Fundamental Theme

• Exploring capabilities and limitations of C
  programs

20
Restating the Fundamental Theme

• We will study the capabilities and limits of C
  programs
• Specifically, we will try and identify
• What problems can be solved by C programs
• What problems cannot be solved by C programs

21
Question

• Is C general enough?
• Or is it possible that there exists some problem
  P such that
• P can be solved by some program P in some other
  reasonable programming language
• but P cannot be solved by any C program?

22
Churchs Thesis (modified)

• We have no proof of an answer, but it is commonly
  accepted that the answer is no.
• Churchs Thesis (three identical statements)
• C is a general model of computation
• Any algorithm can be expressed as a C program
• If some algorithm cannot be expressed by a C
  program, it cannot be expressed in any reasonable
  programming language

23
Summary

• Problems
• When we talk about what programs can or cannot
  DO, we mean what PROBLEMS can or cannot be
  solved

24
Module 3 Classifying Problems

• One of the main themes of this course will be to
  classify problems in various ways
• By solvability
• Solvable, half-solvable, unsolvable
• We will focus our study on decision problems
• function (one correct answer for every input)
• finite range (yes or no is the correct output)

25
Classification Process

• Take some set of problems and partition it into
  two or more subsets of problems where membership
  in a subset is based on some shared problem
  characteristic

26
Classify by Solvability

• Criteria used is whether or not the problem is
  solvable
• that is, does there exist a C program which
  solves the problem?

27
Function Problems

• We will focus on problems where the mapping from
  input to output is a function

28
General (Relation) Problem

• the mapping is a relation
• that is, more than one output is possible for a
  given input

29
Criteria for Function Problems

• mapping is a function
• unique output for each input

30
Example Non-Function Problem

• Divisor Problem
• Input Positive integer n
• Output A positive integral divisor of n

9
31
Example Function Problems

• Sorting
• Multiplication Problem
• Input 2 integers x and y
• Output xy

2,5
32
Another Example

• Maximum divisor problem
• Input Positive integer n
• Output size of maximum divisor of n smaller than
  n

9
33
Decision Problems

• We will focus on function problems where the
  correct answer is always yes or no

34
Criteria for Decision Problems

• Output is yes or no
• range Yes, No
• Note, problem must be a function problem
• only one of Yes/No is correct

35
Example

• Decision sorting
• Input list of integers
• Yes/No question Is the list in nondecreasing
  order?

36
Another Example

• Decision multiplication
• Input Three integers x, y, z
• Yes/No question Is xy z?

37
A Third Example

• Decision Divisor Problem
• Input Two integers x and y
• Yes/No question Is y a divisor of x?

38
Focus on Decision Problems

• When studying solvability, we are going to focus
  specifically on decision problems
• There is no loss of generality, but we will not
  explore that here

39
Finite Domain Problems

• These problems have only a finite number of inputs

40
Lack of Generality

• All finite domain problems can be solved using
  table lookup idea

41
Table Lookup Program

• int main(string x)
• switch x
• case Bill return(3)
• case Judy return(25)
• case Tom return(30)
• default cerr ltlt Illegal input\n

42
Key Concepts

• Classification Theme
• Decision Problems
• Important subset of problems
• We can focus our attention on decision problems
  without loss of generality
• Same is not true for finite domain problems
• Table lookup

43
Module 4 Formal Definition of Solvability

• Analysis of decision problems
• Two types of inputsyes inputs and no inputs
• Language recognition problem
• Analysis of programs which solve decision
  problems
• Four types of inputs yes, no, crash, loop inputs
• Solving and not solving decision problems
• Classifying Decision Problems
• Formal definition of solvable and unsolvable
  decision problems

44
Analyzing Decision Problems

• Can be defined by two sets

45
Decision Problems and Sets

• Decision problems consist of 3 sets
• The set of legal input instances (or universe of
  input instances)
• The set of yes input instances
• The set of no input instances

46
Redundancy

• Only two of these sets are needed the third is
  redundant
• Given
• The set of legal input instances (or universe of
  input instances)
• This is given by the description of a typical
  input instance
• The set of yes input instances
• This is given by the yes/no question
• We can compute
• The set of no input instances

47
Typical Input Universes

• S The set of all finite length strings over
  finite alphabet S
• Examples
• a /\, a, aa, aaa, aaaa, aaaaa,
• a,b /\, a, b, aa, ab, ba, bb, aaa, aab, aba,
  abb,
• 0,1 /\, 0, 1, 00, 01, 10, 11, 000, 001, 010,
  011,
• The set of all integers
• If the input universe is understood, a decision
  problem can be specified by just giving the set
  of yes input instances

48
Language Recognition Problem

• Input Universe
• S for some finite alphabet S
• Yes input instances
• Some set L subset of S
• No input instances
• S - L
• When S is understood, a language recognition
  problem can be specified by just stating what L
  is.

49
Language Recognition Problem

• Traditional Formulation
• Input
• A string x over some finite alphabet S
• Task
• Is x in some language L subset of S?

• 3 set formulation
• Input Universe
• S for a finite alphabet S
• Yes input instances
• Some set L subset of S
• No input instances
• S - L
• When S is understood, a language recognition
  problem can be specified by just stating what L
  is.

50
Equivalence of Decision Problems and Languages

• All decision problems can be formulated as
  language recognition problems
• Simply develop an encoding scheme for
  representing all inputs of the decision problem
  as strings over some fixed alphabet S
• The corresponding language is just the set of
  strings encoding yes input instances
• In what follows, we will often use decision
  problems and languages interchangeably

51
Visualization
52
Analyzing Programs which Solve Decision Problems

• Four possible outcomes

53
Program Declaration

• Suppose a program P is designed to solve some
  decision problem P. What does Ps declaration
  look like?
• What should P return on a yes input instance?
• What should P return on a no input instance?

54
Program Declaration II

• Suppose a program P is designed to solve a
  language recognition problem P. What does Ps
  declaration look like?
• bool main(string x)
• We will assume that the string declaration is
  correctly defined for the input alphabet S
• If S a,b, then string will define variables
  consisting of only as and bs
• If S a, b, , z, A, , Z, then string will
  define variables consisting of any string of
  alphabet characters

55
Programs and Inputs

• Notation
• P denotes a program
• x denotes an input for program P
• 4 possible outcomes of running P on x
• P halts and says yes P accepts input x
• P halts and says no P rejects input x
• P halts without saying yes or no P crashes on
  input x
• We typically ignore this case as it can be
  combined with rejects
• P never halts P infinite loops on input x

56
Programs and the Set of Legal Inputs

• Based on the 4 possible outcomes of running P on
  x, P partitions the set of legal inputs into 4
  groups
• Y(P) The set of inputs P accepts
• When the problem is a language recognition
  problem, Y(P) is often represented as L(P)
• N(P) The set of inputs P rejects
• C(P) The set of inputs P crashes on
• I(P) The set of inputs P infinite loops on
• Because L(P) is often used in place of Y(P) as
  described above, we use notation I(P) to
  represent this set

57
Illustration
All Inputs
I(P)
58
Analyzing Programs and Decision Problems

• Distinguish the two carefully

59
Program solving a decision problem

• Formal Definition
• A program P solves decision problem P if and only
  if
• The set of legal inputs for P is identical to the
  set of input instances of P
• Y(P) is the same as the set of yes input
  instances for P
• N(P) is the same as the set of no input instances
  for P
• Otherwise, program P does not solve problem P
• Note C(P) and I(P) must be empty in order for P
  to solve problem P

60
Solvable Problem

• A decision problem P is solvable if and only if
  there exists some C program P which solves P
• When the decision problem is a language
  recognition problem for language L, we often say
  that L is solvable or L is decidable
• A decision problem P is unsolvable if and only if
  all C programs P do not solve P
• Similar comment as above

61
Illustration of Solvability
Inputs of Program P
Y(P)
N(P)
62
Program half-solving a problem

• Formal Definition
• A program P half-solves problem P if and only if
• The set of legal inputs for P is identical to the
  set of input instances of P
• Y(P) is the same as the set of yes input
  instances for P
• N(P) union C(P) union I(P) is the same as the set
  of no input instances for P
• Otherwise, program P does not half-solve problem
  P
• Note C(P) and I(P) need not be empty

63
Half-solvable Problem

• A decision problem P is half-solvable if and only
  if there exists some C program P which
  half-solves P
• When the decision problem is a language
  recognition problem for language L, we often say
  that L is half-solvable
• A decision problem P is not half-solvable if and
  only if all C programs P do not half-solve P

64
Illustration of Half-Solvability
Inputs of Program P
Y(P)
N(P)
65
Hierarchy of Decision Problems
All decision problems
The set of half-solvable decision problems is a
proper subset of the set of all decision
problems The set of solvable decision problems is
a proper subset of the set of half-solvable
decision problems.
66
Why study half-solvable problems?

• A correct program must halt on all inputs
• Why then do we define and study half-solvable
  problems?
• One Answer the set of half-solvable problems is
  the natural class of problems associated with
  general computational models like C
• Every program half-solves some decision problem
• Some programs do not solve any decision problem
• In particular, programs which do not halt do not
  solve their corresponding decision problems

67
Key Concepts

• Four possible outcomes of running a program on an
  input
• The four subsets every program divides its set of
  legal inputs into
• Formal definition of
• a program solving (half-solving) a decision
  problem
• a problem being solvable (half-solvable)
• Be precise with the above two statements!

68
Module 5

• Topics
• Proof of the existence of unsolvable problems
• Proof Technique
• There are more problems/languages than there are
  programs/algorithms
• Countable and uncountable infinities

69
Overview

• We will show that there are more problems than
  programs
• Actually more problems than programs in any
  computational model (programming language)
• Implication
• Some problems are not solvable

70
Preliminaries

• Define set of problems
• Observation about programs

71
Define set of problems

• We will restrict the set of problems to be the
  set of language recognition problems over the
  alphabet a.
• That is
• Universe a
• Yes Inputs Some language L subset of a
• No Inputs a - L

72
Set of Problems

• The number of distinct problems is given by the
  number of languages L subset of a
• 2a is our shorthand for this set of subset
  languages
• Examples of languages L subset of a
• 0 elements
• 1 element /\, a, aa, aaa, aaaa,
• 2 elements /\, a, /\, aa, a, aa,
• Infinite of elements an n is even, an n
  is prime, an n is a perfect square

73
Infinity and a

• All strings in a have finite length
• The number of strings in a is infinite
• The number of languages L in 2a is infinite
• The number of strings in a language L in 2a
  may be finite or infinite

74
Define set of programs

• The set of programs we will consider are the set
  of legal C programs as defined in earlier
  lectures
• Key Observation
• Each C program can be thought of as a finite
  length string over alphabet SP
• SP a, , z, A, , Z, 0, , 9, white space,
  punctuation

75
Example

• int main(int A, int n) 26 characters
  including newline
• int i, max 13
  characters including initial tab
• 
  1 character newline
• if (n lt 1) 12
  characters
• return (Illegal Input) 28 characters
  including 2 tabs
• max A0 13
  characters
• for (i 1 i lt n i) 25
  characters
• if (Ai gt max) 18
  characters
• max Ai 15
  characters
• return (max) 15
  characters
• 2
  characters including newline

76
Number of programs

• The set of legal C programs is clearly infinite
• It is also no more than SP
• SP a, , z, A, , Z, 0, , 9, white space,
  punctuation

77
Goal

• Show that the number of languages L in 2a is
  greater than the number of strings in SP
• SP a, , z, A, , Z, 0, , 9, white space,
  punctuation
• Problem
• Both are infinite

78
How do we compare the relative sizes of infinite
sets?

• Bijection (yes)
• Proper subset (no)

79
Bijections

• Two sets have EQUAL size if there exists a
  bijection between them
• bijection is a 1-1 and onto function between two
  sets
• Examples
• Set 1, 2, 3 and Set A, B, C
• Positive even numbers and positive integers

80
Bijection Example

• Positive Integers Positive Even Integers
• 1 2
• 2 4
• 3 6
• ... ...
• i 2i
• ...

81
Proper subset

• Finite sets
• S1 proper subset of S2 implies S2 is strictly
  bigger than S1
• Example
• women proper subset of people
• number of women less than number of people
• Infinite sets
• Counterexample
• even numbers and integers

82
Two sizes of infinity

• Countable
• Uncountable

83
Countably infinite set S

• Definition 1
• S is equal in size (bijection) to N
• N is the set of natural numbers 1, 2, 3,
• Definition 2 (Key property)
• There exists a way to list all the elements of
  set S (enumerate S) such that the following is
  true
• Every element appears at a finite position in the
  infinite list

84
Uncountable infinity

• Any set which is not countably infinite
• Examples
• Set of real numbers
• 2a, the set of all languages L which are a
  subset of a
• Further gradations within this set, but we ignore
  them

85
Proof
86
(1) The set of all legal C programs is
countably infinite

• Every C program is a finite string
• Thus, the set of all legal C programs is a
  language LC
• This language LC is a subset of SP

87
For any alphabet S, ? is countably infinite

• Enumeration ordering
• All length 0 strings
• S0 1 string l
• All length 1 strings
• S strings
• All length 2 strings
• S2 strings
• Thus, SP is countably infinite

88
Example with alphabet a,b

• Length 0 strings
• 0 and l
• Length 1 strings
• 1 and a, 2 and b
• Length 2 strings
• 3 and aa, 4 and ab, 5 and ba, 6 and bb, ...
• Question
• write a program that takes a number as input and
  computes the corresponding string as output

89
(2) The set of languages in 2a is uncountably
infinite

• Diagonalization proof technique
• Algorithmic proof
• Typically presented as a proof by contradiction

90
Algorithm Overview

• To prove this set is uncountably infinite, we
  construct an algorithm D that behaves as follows
• Input
• A countably infinite list of languages L subset
  of a
• Output
• A language D(L) which is a subset of a that
  is not on list L

91
Visualizing D
List L L0 L1 L2 L3 ...
Language D(L) not in list L
92
Why existence of D implies result

• If the number of languages in 2a is countably
  infinite, there exists a list L s.t.
• L is complete
• it contains every language in 2a
• L is countably infinite
• The existence of algorithm D implies that no list
  of languages in 2a is both complete and
  countably infinite
• Specifically, the existence of D shows that any
  countably infinite list of languages is not
  complete

93
Visualizing One Possible L
l
a
aa
aaa
aaaa
...

• Rows is countably infinite
• Given
• Cols is countably infinite
• a is countably infinite

L0
L1
L2
L3
L4
...

• Consider each string to be a feature
• A set contains or does not contain each string

94
Constructing D(L )

• We construct D(L) by using a unique feature
  (string) to differentiate D(L) from Li
• Typically use ith string for language Li
• Thus the name diagonalization

D(L)
l
a
aa
aaa
aaaa
...
OUT
L0
IN
IN
IN
IN
IN
L1
OUT
IN
IN
IN
OUT
IN
L2
OUT
OUT
OUT
OUT
OUT
IN
L3
IN
IN
OUT
OUT
OUT
IN
L4
IN
IN
OUT
OUT
OUT
OUT
...
95
D(L) cannot be any language Li

• D(L) cannot be language L0 because D(L)
  differs from L0 with respect to string a0 /\.
• If L0 contains /\, then D(L) does not, and
  vice versa.
• D(L) cannot be language L1 because D(L)
  differs from L1 with respect to string a1 a.
• If L1 contains a, then D(L) does not, and
  vice versa.
• D(L) cannot be language L2 because D(L)
  differs from L2 with respect to string a2 aa.
• If L2 contains aa, then D(L) does not, and
  vice versa.

96
Questions
l
a
aa
aaa
aaaa
...
L0
IN
IN
IN
IN
IN
L1
OUT
IN
IN
IN
OUT
L2
OUT
OUT
OUT
OUT
OUT
L3
IN
IN
OUT
OUT
OUT
L4
IN
IN
OUT
OUT
OUT
...

• Do we need to use the diagonal?
• Every other column and every row?
• Every other row and every column?
• What properties are needed to construct D(L)?

97
Visualization
All problems
The set of solvable problems is a proper subset
of the set of all problems.
98
Summary

• Equal size infinite sets bijections
• Countable and uncountable infinities
• More languages than algorithms
• Number of algorithms countably infinite
• Number of languages uncountably infinite
• Diagonalization technique
• Construct D(L) using infinite set of features
• The set of solvable problems is a proper subset
  of the set of all problems

99
Module 6

• Topics
• Program behavior problems
• Input of problem is a program/algorithm
• Definition of type program
• Program correctness
• Testing versus Proving

100
Number Theory Problems

• These are problems where we investigate
  properties of numbers
• Primality
• Input Positive integer n
• Yes/No Question Is n a prime number?
• Divisor
• Input Integers m,n
• Yes/No question Is m a divisor of n?

101
Graph Theory Problems

• These are problems where we investigate
  properties of graphs
• Connected
• Input Graph G
• Yes/No Question Is G a connected graph?
• Subgraph
• Input Graphs G1 and G2
• Yes/No question Is G1 a subgraph of G2?

102
Program Behavior Problems

• These are problems where we investigate
  properties of programs and how they behave
• Give an example problem with one input program P
• Give an example problem with two input programs
  P1 and P2

103
Program Representation

• Program variables
• Abstractly, we define the type program
• graph G, program P
• More concretely, we define type program to be a
  string over the program alphabet SP a, , z,
  A, , Z, 0, , 9, punctuation, white space
• Note, many strings over SP are not legal programs
  
• We consider them to be programs that always crash
• Possible declaration of main procedure
• bool main(program P)

104
Program correctness

• How do we determine whether or not a program P we
  have written is correct?
• What are some weaknesses of this approach?
• What might be a better approach?

105
Testing versus Analyzing
Test Inputs x1 x2 x3 ...
Outputs P(x1) P(x2) P(x3) ...
Analysis of Program P
Program P
106
2 Program Behavior Problems

• Correctness
• Input
• Program P
• Yes/No Question
• Does P correctly solve the primality problem?
• Functional Equivalence
• Input
• Programs P1, P2
• Yes/No Question
• Is program P1 functionally equivalent to program
  P2

107
Module 7

• Halting Problem
• Fundamental program behavior problem
• A specific unsolvable problem
• Diagonalization technique revisited
• Proof more complex

108
Definition

• Input
• Program P
• Assume the input to program P is a single
  nonnegative integer
• This assumption is not necessary, but it
  simplifies the following unsolvability proof
• To see the full generality of the halting
  problem, remove this assumption
• Nonnegative integer x, an input for program P
• Yes/No Question
• Does P halt when run on x?
• Notation
• Use H as shorthand for halting problem when space
  is a constraint

109
Example Input

• Program with one input of type unsigned
• bool main(unsigned Q)
• int i2
• if ((Q 0) (Q 1)) return false
• while (iltQ)
• if (Qi 0) return (false)
• i
• return (true)
• Input x
• 4

110
Three key definitions
111
Definition of list L

• SP is countably infinite where SP
  characters, digits, white space, punctuation
• Type program will be type string with SP as the
  alphabet
• Define L to be the strings in SP listed in
  enumeration order
• length 0 strings first
• length 1 strings next
• Every program is a string in SP
• For simplicity, consider only programs that have
• one input
• the type of this input is an unsigned
• Consider strings in SP that are not legal
  programs to be programs that always crash (and
  thus halt on all inputs)

112
Definition of PH

• If H is solvable, some program must solve H
• Let PH be a procedure which solves H
• We declare it as a procedure because we will use
  PH as a subroutine
• Declaration of PH
• bool PH(program P, unsigned x)
• In general, the type of x should be the type of
  the input to P
• Comments
• We do not know how PH works
• However, if H is solvable, we can build programs
  which call PH as a subroutine

113
Definition of program D

• bool main(unsigned y) / main for program D /
• program P generate(y)
• if (PH(P,y)) while (1gt0) else return (yes)
• / generate the yth string in SP in enumeration
  order /
• program generate(unsigned y)
• / code for program of slide 21 from module 5
  did this for a,b /
• bool PH(program P, unsigned x)
• / how PH solves H is unknown /

114
Generating Py from y

• We wont go into this in detail here
• This was the basis of the question at the bottom
  of slide 21 of lecture 5 (alphabet for that
  problem was a,b instead of SP).
• This is the main place where our assumption about
  the input type for program P is important
• for other input types, how to do this would vary
• Specification
• 0 maps to program l
• 1 maps to program a
• 2 maps to program b
• 3 maps to program c
• 26 maps to program z
• 27 maps to program A

115
Proof that H is not solvable
116
Argument Overview
H is solvable
D is NOT on list L
p ? q is logically equivalent to (not q) ? (not p)
117
Proving D is not on list L

• Use list L to specify a program behavior B that
  is distinct from all real program behaviors (for
  programs with one input of type unsigned)
• Diagonalization argument similar to the one for
  proving the number of languages over a is
  uncountably infinite
• No program P exists that exhibits program
  behavior B
• Argue that D exhibits program behavior B
• Thus D cannot exist and thus is not on list L

118
Non-existent program behavior B
119
Visualizing List L
0
1
2
3
4
...

• Rows is countably infinite
• Sp is countably infinite
• Cols is countably infinite
• Set of nonnegative integers is countably infinite

P0
P1
P2
P3
P4
...

• Consider each number to be a feature
• A program halts or doesnt halt on each integer
• We have a fixed L this time

120
Diagonalization to specify B

• We specify a non-existent program behavior B by
  using a unique feature
• (number) to differentiate B from Pi

0
1
2
3
4
...
B
P0
NH
H
H
H
H
H
P1
NH
H
H
H
NH
H
P2
NH
NH
NH
NH
NH
H
P3
H
H
NH
NH
NH
H
P4
NH
H
H
H
H
H
...
121
Arguing D exhibits program behavior B
122
Code for D

• bool main(unsigned y) / main for program D /
• program P generate(y)
• if (PH(P,y)) while (1gt0) else return (yes)
• / generate the yth string in SP in enumeration
  order /
• program generate(unsigned y)
• / code for extra credit program of slide 21
  from lecture 5 did this for a,b /
• bool PH(program P, unsigned x)
• / how PH solves H is unknown /

123
Visualization of D in action on input y

• Program D with input y
• (type for y unsigned)
• Given input y, generate the program (string) Py
• Run PH on Py and y
• Guaranteed to halt since PH solves H
• IF (PH(Py,y)) while (1gt0) else return (yes)

0
1
2
...
D
...
y
P0
H
H
H
P1
H
H
NH
P2
NH
NH
NH
...
Py
H
NH
...
124
D cannot be any program on list L

• D cannot be program P0 because D behaves
  differently on 0 than P0 does on 0.
• If P0 halts on 0, then D does not, and vice
  versa.
• D cannot be program P1 because D behaves
  differently on 1 than P1 does on 1.
• If P1 halts on 1, then D does not, and vice
  versa.
• D cannot be program P2 because D behaves
  differently on 2 than P2 does on 2.
• If P2 halts on 2, then D does not, and vice
  versa.

125
Alternate Proof
126
Alternate Proof Overview

• For every program Py, there is a number y that we
  associate with it
• The number we use to distinguish program Py from
  D is this number y
• Using this idea, we can arrive at a contradiction
  without explicitly using the table L
• The diagonalization is hidden

127
H is not solvable, proof II

• Assume H is solvable
• Let PH be the program which solves H
• Use PH to construct program D which cannot exist
• Contradiction
• This means program PH cannot exist.
• This implies H is not solvable
• D is the same as before

128
Arguing D cannot exist

• If D is a program, it must have an associated
  number y
• What does D do on this number y?
• 2 cases
• D halts on y
• This means PH(D,y) NO
• Definition of D
• This means D does not halt on y
• PH solves H
• Contradiction
• This case is not possible

129
Continued

• D does not halt on this number y
• This means PH(D,y) YES
• Definition of D
• This means D halts on y
• PH solves H
• Contradiction
• This case is not possible
• Both cases are not possible, but one must be for
  D to exist
• Thus D cannot exist

130
Placing the Halting Problem
All Problems
Solvable
131
Implications

• The Halting Problem is one of the simplest
  problems we can formulate about program behavior
• We can use the fact that it is unsolvable to show
  that other problems about program behavior are
  also unsolvable
• This has important implications restricting what
  we can do in the field of software engineering
• In particular, perfect debuggers/testers do not
  exist
• We are forced to test programs for correctness
  even though this approach has many flaws

132
Summary

• Halting Problem definition
• Basic problem about program behavior
• Halting Problem is unsolvable
• We have identified a specific unsolvable problem
• Diagonalization technique
• Proof more complicated because we actually need
  to construct D, not just give a specification B

133
Module 8

• Halting Problem revisited
• Universal Turing machine half-solves halting
  problem
• A universal Turing machine is an operating
  system/general purpose computer

134
Half-solving the Halting Problem

• State the Halting Problem
• Give an input instance of the Halting Problem
• We saw last time that the Halting Problem is not
  solvable. How might we half-solve the Halting
  Problem?

135
Example Input

• Program P
• bool main(unsigned Q)
• int i2
• if ((Q 0) (Q 1)) return false
• while (iltQ)
• if (Qi 0) return (false)
• i
• return (true)
• Nonnegative integer
• 4

136
Organization
Universal Turing machines Memory
Program P bool main(unsigned Q) int i2 if ((Q
0) (Q 1)) return false while (iltQ)
if (Qi 0) return (false) i return
(true)
int i,Q
4
Line 1
137
Description of universal Turing machine

• Basic Loop
• Find current line of program P
• Execute current line of program P
• Update program Ps memory
• Update program counter
• Return to Top of Loop

138
Past, Present, Future

• Turing came up with the concept of a universal
  Turing machine in the 1930s
• This is well before the invention of the general
  purpose computer
• People were still thinking of computing devices
  as special-purpose devices (calculators, etc.)
• Turing helped move people beyond this narrow
  perspective
• Turing/Von Neumann perspective
• Computers are general purpose/universal
  algorithms
• Focused on computation
• Stand-alone
• Today, we are moving beyond this view
• Internet, world-wide web
• However, results in Turing perspective still
  relevant

139
Debuggers

• How are debuggers like gdb or ddd related to
  universal Turing machines?
• How do debuggers simplify the debugging process?

140
Placing the Halting Problem
All Problems
Half-solvable
Solvable
141
Summary

• Universal Turing machines
• 1930s, Turing
• Introduces general purpose computing concept
• Not a super intelligent program, merely a precise
  follower of instructions
• Halting Problem half-solvable but not solvable

142
Module 9

• Closure Properties
• Definition
• Language class definition
• set of languages
• Closure properties and first-order logic
  statements
• For all, there exists

143
Closure Properties

• A set is closed under an operation if applying
  the operation to elements of the set produces
  another element of the set
• Example/Counterexample
• set of integers and addition
• set of integers and division

144
Integers and Addition
7
Integers
145
Integers and Division
.4
2
5
Integers
146
Language Classes

• We will be interested in closure properties of
  language classes
• A language class is a set of languages
• Thus, the elements of a language class (set of
  languages) are languages which are sets
  themselves
• Crucial Observation
• When we say that a language class is closed under
  some set operation, we apply the set operation to
  the languages (elements of the language classes)
  rather than the language classes themselves

147
Example Language Classes

• In all these examples, we do not explicitly state
  what the underlying alphabet S is
• Solvable languages (REC)
• Languages whose language recognition problem can
  be solved
• Half-solvable languages (RE)
• Languages whose language recognition problems can
  be half-solved
• Finite languages
• Languages with a finite number of strings
• CARD-3
• Languages with at most 3 strings

148
Finite Sets and Set Union
a,b,aa,bb
Finite Languages
149
CARD-3 and Set Union
a,b,aa,bb
CARD-3
150
Finite Languages and Set Complement
/\,aa,ba,bb,aaa,...
a,b,ab
Finite Languages
151
Infinite Number of Facts

• A closure property often represents an infinite
  number of facts
• Example The set of finite languages is closed
  under the set union operation
• union is a finite language
• union l is a finite language
• union 0 is a finite language
• ...
• l union is a finite language
• ...

152
First-order logic and closure properties

• A way to formally write (not prove) a closure
  property
• " L1, ...,Lk in LC, op (L1, ... Lk) in LC
• Only one expression is needed because of the for
  all quantifier
• Number of languages k is determined by arity of
  the operation op

153
Example F-O logic statements

• " L1,L2 in FINITE, L1 union L2 in FINITE
• " L1,L2 in CARD-3, L1 union L2 in CARD-3
• " L in FINITE, Lc in FINITE
• " L in CARD-3, Lc in CARD-3

154
Stating a closure property is false

• What is true if a set is not closed under some
  k-ary operator?
• There exist k elements of that set which, when
  combined together under the given operator,
  produce an element not in the set
• L1, ...,Lk in LC, op (L1, , Lk) not in LC
• Example
• Finite sets and set complement

155
Complementing a F-O logic statement

• Complement " L1,L2 in CARD-3, L1 union L2 in
  CARD-3
• not (" L1,L2 in CARD-3, L1 union L2 in CARD-3)
• L1,L2 in CARD-3, not (L1 union L2 in CARD-3)
• L1,L2 in CARD-3, L1 union L2 not in CARD-3

156
Proving/Disproving

• Which is easier and why?
• Proving a closure property is true
• Proving a closure property is false

157
Module 10

• Recursive and r.e. language classes
• representing solvable and half-solvable problems
• Proofs of closure properties
• for the set of recursive (solvable) languages
• for the set of r.e. (half-solvable) languages
• Generic Element proof technique

158
RE and REC language classes

• REC
• A solvable language is commonly referred to as a
  recursive language for historical reasons
• REC is defined to be the set of solvable or
  recursive languages
• RE
• A half-solvable language is commonly referred to
  as a recursively enumerable or r.e. language
• RE is defined to be the set of r.e. or
  half-solvable languages

159
Closure Properties of REC

• We now prove REC is closed under two set
  operations
• Set Complement
• Set Intersection
• In these proofs, we try to highlight intuition
  and common sense

160
REC and Set Complement

• Even set of even length strings
• Is Even solvable (recursive)?
• Give a program P that solves it.
• Complement of Even?
• Odd set of odd length strings
• Is Odd recursive (solvable)?
• Does this prove REC is closed under set
  complement?
• How is the program P that solves Odd related to
  the program P that solves Even?

REC
EVEN
All Languages
161
P Illustration
P
Yes/No
Input x
162
Code for P

• bool main(string y)
• if (P (y)) return no else return yes
• bool P (string y)
• / details deleted key fact is P is guaranteed
  to halt on all inputs /

163
Set Complement Lemma

• If L is a solvable language, then L complement is
  a solvable language
• Proof
• Let L be an arbitrary solvable language
• First line comes from For all L in REC
• Let P be the C program which solves L
• P exists by definition of REC

164
proof continued

• Modify P to form P as follows
• Identical except at very end
• Complement answer
• Yes ? No
• No ? Yes
• Program P solves L complement
• Halts on all inputs
• Answers correctly
• Thus L complement is solvable
• Definition of solvable

165
REC Closed Under Set Union

• If L1 and L2 are solvable languages, then L1 U L2
  is a solvable language
• Proof
• Let L1 and L2 be arbitrary solvable languages
• Let P1 and P2 be programs which solve L1 and L2,
  respectively

L1 U L2
REC
All Languages
166
REC Closed Under Set Union

• Construct program P3 from P1 and P2 as follows
• P3 solves L1 U L2
• Halts on all inputs
• Answers correctly
• L1 U L2 is solvable

L1 U L2
REC
All Languages
167
P3 Illustration
Yes/No
P1
Yes/No
P2
168
Code for P3

• bool main(string y)
• if (P1(y)) return yes
• else if (P2(y)) return yes
• else return no
• bool P1(string y) / details deleted key fact
  is P1 always halts. /
• bool P2(string y) / details deleted key fact is
  P2 always halts. /

169
Other Closure Properties

• Unary Operations
• Language Reversal
• Kleene Star
• Binary Operations
• Set Intersection
• Set Difference
• Symmetric Difference
• Concatenation

170
Closure Properties of RE

• We now try to prove RE is closed under the same
  two set operations
• Set Union
• Set Complement
• In these proofs
• We define a more formal proof methodology
• We gain more intuition about the differences
  between solvable and half-solvable problems

171
RE Closed Under Set Union

• Expressing this closure property as an infinite
  set of facts
• Let Li denote the ith r.e. language
• L1 intersect L1 is in RE
• L1 intersect L2 is in RE
• ...
• L2 intersect L1 is in RE
• ...

172
Generic Element or Template Proofs

• Since there are an infinite number of facts to
  prove, we cannot prove them all individually
• Instead, we create a single proof that proves
  each fact simultaneously
• I like to call these proofs generic element or
  template proofs

173
Basic Proof Ideas

• Name your generic objects
• For example, L or L1 or L2
• Only use facts which apply to any relevant
  objects
• For example, there must exist a program P1 that
  half-solves L1
• Work from both ends of the proof
• The first and last lines are often obvious, and
  we can often work our way in

174
RE Closed Under Set Union

• Let L1 and L2 be arbitrary r.e. languages

• There exist P1 and P2 s.t. Y(P1)L1 and Y(P2)L2

• Construct program P3 from P1 and P2
• Prove Program P3 half-solves L1 U L2

• There exists a program P that half-solves L1 U L2

• L1 U L2 is an r.e. language

175
Constructing Program P3

• What code did we use for P3 when we worked with
  solvable languages?

bool main(string y) if (P1(y)) return yes
else if (P2(y)) return yes else return
no bool P1(string y) / details deleted key
fact is P1 always halts. / bool P2(string y) /
details deleted key fact is P2 always halts. /

• Will this code work for half-solvable languages?

176
Proving P3 Is Correct

• 2 steps to showing P3 half-solves L1 U L2
• For all x in L1 U L2, must show P3
• For all x not in L1 U L2, must show P3

177
P3 works correctly?
bool main(string y) if (P1(y)) return yes
else if (P2(y)) return yes else return no

• Let x be an arbitrary string in L1 U L2
• Note, this subproof is a generic element proof
• What are the two possibilities for x?
• x is in L1
• x is in L2
• What does P3 do if x is in L1?
• Does it matter if x is in or not in L2?
• What does P3 do if x is in L2?
• Does it matter if x is in or not in L1?
• Does P3 work correctly on such x?
• If not, what strings cause P3 a problem?

178
P3 works correctly?
bool main(string y) if (P1(y)) return yes
else if (P2(y)) return yes else return no

• Let x be an arbitrary string NOT in L1 U L2
• Note, this subproof is a generic element proof
• x is not in L1 AND x is not in L2
• What does P3 do on x in this case?
• What does P1 do on x?
• What does P2 do on x?
• Does P3 work correctly on such x?

179
Code for Correct P3

• bool main(string y)
• if ((P1(y) P2(y)) return yes
• / P1 and P2 run in parallel (alternating
  execution) /
• else return no
• bool P1(string y)
• / key fact is P1 only guaranteed to halt on yes
  input instances /
• bool P2(string y)
• / key fact is P2 only guaranteed to halt on yes
  input instances /

180
P3 works correctly?
bool main(string y) if ((P1(y) P2(y))
return yes else return no / P1 and P2
run in parallel /

• Let x be an arbitrary string in L1 U L2
• Note, this subproof is a generic element proof
• What are the two possibilities for x?
• x is in L1
• x is in L2
• What does P3 do if x is in L1?
• Does it matter if x is in or not in L2?
• What does P3 do if x is in L2?
• Does it matter if x is in or not in L1?
• Does P3 work correctly on such x?
• If not, what strings