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Title: Computing Fundamentals 1 Lecture 6 A Theory of Sets


1
Computing Fundamentals 1 Lecture 6A Theory of
Sets
  • Lecturer Patrick Browne
  • http//www.comp.dit.ie/pbrowne/
  • Room K308
  • Based on Chapter 11.
  • A Logical approach to Discrete Math
  • By David Gries and Fred B. Schneider

2
Sets
  • Set theory can be defined as an extension of
    predicate calculus. A set is a collection of
    distinct elements (e.g. set of integers, set of
    students).
  • The operations on sets (e.g. union) correspond in
    a natural way to the logical connectives of logic
    (e.g. or). Later we will show how DeMorgans Laws
    for logic can be applied to sets.

3
Sets
  • Two methods of describing sets set enumeration
    and set comprehension.
  • Set comprehension is defined in terms testing
    membership in sets.
  • This membership test is the basis for the
    definition of equality of sets.

4
Sets
  • Enumerations list elements
  • Constants 1,2,3,
  • Variables a,b
  • Comprehension state properties
  • x? 0 ? x lt 5 2 ? x
  • Represents
  • 0,2,4,6,8
  • The general form of comprehension (11.1) is
  • xt R E

5
Sets
  • Recall from Lecture 1 a state is a list of
    variables with associated values.
  • The evaluation of
  • xt RE
  • in a state yields the set of values from
    evaluating Ex v in the state for each value
    v in t such that Rxv holds in that state.
    Notation similar to quantification

6
Universe
  • A theory of sets concerns sets constructed from
    some collection of elements. There is a theory of
    sets of integers, a theory of sets of characters,
    a theory of sets of sets of integers. The
    collection of elements is called the domain of
    discourse or universe of values and is denoted by
    U. The DOD can be thought of as the type of every
    set variable in the theory (e.g. DOD is set(?),
    then vset(?)).

7
Universe
  • When several set theories are being used at the
    same time, there is a different universe for
    each. The name U is then overloaded similar to 1
    being considered as a real or an integer.

8
Set Membership Equality
  • For expression e and a set valued expression S
  • e ? S
  • is an expression whose value is the value of
    the statement e is a member of S. We can write
    e is not a member of S as
  • ?(e ? S) or (e?S)
  • The symbol ? is treated as conjunctional operator
    and has the same precedence as equality ().

9
Set Membership Equality
  • (11.3) Axiom, set membership. For expression Ft
    and set xR Et for type t
  • provided ? occurs(x,F)
  • F ? xR E ? (?x RFE)
  • Example
  • y ? x xgt1 3x) ? (?x xgt1y3x)
  • (11.4) Axiom ,extensionality
  • ST ? (?x x?S ? x?T)
  • An extensional definition depends only on the
    content of the set. In contrast, an intentional1
    definition describes how the set was defined or
    constructed.

10
Sets versus Predicates
  • (11.7) Theorem
  • x ? xRx ? R
  • y ? xR ? Rxy
  • Theorem (11.7) formalizes the connection between
    sets and predicates a predicate is a
    representation for the set of argument-values for
    which the predicate is true.
  • 2 ? xxgt1 ? 2gt1

11
Sets versus Predicates
  • (11.8) The principle of comprehension
  • To each predicate R there corresponds a set
    comprehension xR , which contains the objects
    t that satisfy R R is called the characteristic
    predicate of the set.

12
Sets versus Predicates
  • (11.8) means we can define a set using
    comprehension and by its characteristic
    predicate.
  • For example the set comprehension
  • S x x3 ? x5
  • And the characteristic predicate
  • x ? S ? x3 ? x5 (for all x)
  • Are equivalent.

13
Sets versus Predicates
  • Theorem (11.10)
  • xQxR is valid iff Q ? R is valid.

14
Operations on Sets
15
Operations on Sets
  • Three methods for proving set equality
  • (a) Use Leibniz directly
  • (b) Use axiom of Extensionality (11.4) and prove
    v ? S ? v ? T for an arbitrary value of v.
  • (c) Prove Q ? R and conclude xQxR

16
Operations on Sets
  • Let S,T,U,V have type set(t) for some type t.
    This convention allows us to shorten a definition
  • For example, set cardinality
  • (?Sset(t) S (?x x?S1))
  • can be defined as
  • S (?x x?S1)

17
Operations on Sets
18
Operations on Sets
19
Operations on Sets
20
Operations on Sets
21
EU set
  • Let the set EU be the set of all countries in the
    European Union (old).
  • EU the set of all countries in the European
    Union.
  • It could be declared as an enumeration
  • EUA,B,DK,F,SF,D,GB,GR,IRL,I,L,NL,P,E,S
  • The variable homeland can refer to one element in
    the set EU
  • homeland ? EU
  • beneluxB,NL,L

22
The Empty Set and Singleton
  • It is possible to have a set with no values, it
    is called the empty set and denoted as
  • 0 or
  • A set that contains only one element is called a
    singleton set. For example IRL
  • Note the difference between the set IRL and
    element IRL.
  • Sets are not ordered. There are no duplicates,
    each element is unique.

23
Subset or Set inclusion
The following statements are true B,NL z
B,NL,L 0 zB,NL,L B,NL,L z B,NL,L
X (universal set) SPX (? is power set) TPX S z
T
T
S
X
24
Subset Superset
  • S is a subset of T if every element if every
    element of S is an element of T.
  • (11.13) axiom, subset
  • S ? T ? (?x x ? S x ? T)
  • (11.14) axiom, proper subset
  • S ? T ? S?T ? S?T

25
Subset Superset
  • (11.15) axiom, superset
  • T ? S ? S?T
  • (11.16) axiom, proper superset
  • T ? S ? S ? T
  • The axioms and theorems for set operations are
    given in the course text and the course web site,
    only a few are covered in the slides.

26
Set Operators
  • Set Equivalence
  • Two values of the same type can be tested to see
    if they are the same by using the equals sign, as
    in
  • x y
  • Two sets are equal if they contain exactly the
    same elements. For example the two sets below are
    equal
  • B,NL,L NL, B, L

27
Set Operators
  • Set Non-Equivalence
  • Two values of the same type can be tested to see
    if they are not the same by using the not-equals
    sign ?. Two sets are not equal if they do not
    contain exactly the same elements
  • B,NL ? B,NL,L

28
Set Operators
  • The membership operator is written e
  • NL e B, NL, Lit is true the Netherlands is a
    Benelux country
  • General case below

SPX xX xeS
.x
S
X
29
Set Operators
  • The non-membership operator is written
  • IRL?B, NL, L Ireland is not a Benelux country

SPX xX x ? S
General case
S
.x
X
30
Set Operators
  • The validity of membership test.
  • The value to be tested for membership must be an
    element of the underlying type of the set. For
    example
  • USA e B, NL, L
  • is illegal, since USA is not an element of the
    type EU.

31
Set Operators
  • Size Cardinality The number of values in a set
    is called its size, or cardinality, and is
    signified with the has sign
  • B,NL,L 3
  • IRL 1
  • IRL illegal, IRL is not a set
  • ? 0

32
Set Operators
  • Powersets The powerset of a set is written
  • PS
  • It is the set of all subsets of S. For example
    the powerset of the Benelux countries is
  • PB,NL,L ?,
  • B,NL,L,
  • B,NL,B,L,NL,L,
  • B,NL,L

33
Set OperatorsPowersets
  • When a variable is to be declared to have a type
    that is a set of elements, the type is the
    powerset of the type of the elements
  • benelux PEU
  • This can be read as the variable benelux is a
    subset of the set of countries EU or the
    variable benelux is a set of EU countries

34
Set OperatorsPowersets
  • The size of the powerset of a set is equal to two
    raised to the power of the size of the set.
  • (PS) 2s
  • B,NL,L 3
  • (PB,NL,L) 8

35
Complement
  • The complement of S (S) is the set of elements
    that are not in S but are in the universe.
  • (11.17) axiom, complement
  • v?Sc ? v?U ? v?S

S
U
36
Set Operators Union
  • The union of two sets is the set containing all
    the elements that are in either the first set or
    the second set or both.

X SPX TPX T U S T U S T U S S U 0 S
S
T
X
37
Set Operators Intersection
  • The intersection of two sets is the set
    containing all the elements that are in the first
    set and in the second set.

X SPX TPX T I S T I S S I T S I 0 0
S
T
X
38
Set Operators Difference
  • The difference of two sets is the set containing
    all the elements that are in the first set and
    are not in the second set. The shaded area is the
    difference of S and T.

X SPX TPX S \ T S \ T T \ S S \ 0 S 0
\ S 0
S
T
X
39
Set Operators Distributed Union
  • The distributed union of a set of sets is the set
    containing just those elements that occur in at
    least one of the component sets.

X SPX RPX TPX R,S,T
R
S
T
X
40
Set Operators Distributed Intersection
  • The distributed intersection of a set of sets is
    the set containing just those elements that occur
    in all of the component sets.

X SPX RPX TPX R,S,T
R
T
S
X
41
Disjoint Sets
  • Sets that are disjoint have no elements in
    common their intersection is the empty set.
  • For disjoint sets T and S the following
    expression is true
  • T I S 0.

42
More than 2 Disjoint Sets
  • For more that two sets it becomes longer, since
    every pair must have an empty intersection. For
    example for sets A, B and C to be disjoint the
    following must be true
  • A I B 0 and
  • B I C 0 and
  • C I A 0

43
Partition
  • A sequence of sets is said to partition another
    larger set if the sets are disjoint and their
    distributed union is the entire larger set if the
    sets are disjoint. For example
  • if
  • disjointltA,B,Cgt ? (?A,B,C T)
  • then ltA,B,Cgt partition T
  • For example
  • ltTemporary, Part-time, Permanentgt may partition
    Employee.

44
Relating sets and Boolean Expressions
  • The definition of the set operations reveals a
    connection between the set operators and the
    propositional operators. For example, in the
    definition of ? , as the phrase x? of the LHS
    is distributed inward to the operands S and T of
    the RHS, ? becomes ?.
  • x ? S ? T ? x ? S ? x ? T

45
Relating sets and Boolean Expressions
  • The properties of propositional operators are
    reflected as similar properties of set operators,
    e.g. absorption laws
  • S ? (S ? T) ? S (type S,TBoolean)
  • S ? (S ? T) S (type S,Tset(t))
  • The zero laws for props. sets.
  • S ? false ? false (type SBoolean)
  • S ? 0 0 (type Sset(t))

46
Set expression Es Propositional expression Ep
47
Numerical Problem for 3 sets (see lab 4)
  • Out of group of 42 students
  • 3 studied French, Business and Music.
  • 10 studied French and Business.
  • 6 studied French and Music.
  • 5 studied Business and Music.
  • 18 studied French.
  • 20 studied Business.
  • 17 studied Music.

48
Another Set Problem
  • A group of 41 students study Finance, Business
    and Mathematics as follows
  • 2 studied Finance, Business and Mathematics.
  • 9 studied Finance and Business.
  • 4 studied Finance and Mathematics.
  • 5 studied Business and Mathematics.
  • 17 studied Finance.
  • 20 studied Business.
  • 16 studied Mathematics.

49
Another Set Problem
50
Another Numerical Set Problem
  • How many studied Finance and Mathematics but not
    Business?
  • (Finance Mathematics) All Three
  • 4 - 2 2.

51
Another Set Problem
  • How many studied Business and neither Finance nor
    Mathematics?
  • Business - FinanceBusiness -
    MathematicsBusiness All three
  • 20 5 9 2 8

The overlap (intersection) between these two
pairs was removed twice so it needs to be added
back
52
Another Set Problem
  • How many are taking none of the subjects?
  • 17201653 subjects with intersections
  • (945 ) pair wise intersections
  • 53 - (9 4 5 ) 2 37 remove intersections
    and double counting
  • 41 37 4 not taking a subject.

53
A Set Proof
  • We will prove the distribution of set union over
    set intersection
  • A ? (B ? C) (A?B) ? (A?C)
  • We will use the following theorem and axiom
  • (1) p ?(q?r) ? (p?q)?(p?r)
  • (2) S T ? (?v v ? S ? v ? T)
  • And the definitions of union and intersection
  • (3) v ? B ? C ? v ? B ? v ? C
  • (4) v ? B ? C ? v ? B ? v ? C

54
A Set Proof
  • Axiom are numbered as in the previous slide. We
    prove distribution by using Extensionality (3) to
    show that an arbitrary element v is in LHS
    exactly when it is in the RHS.
  • v ? A ? (B ? C)
  • lt definition of ? (3) gt
  • v ? A ? v ? (B ? C)
  • lt definition of ? (4) gt
  • v ? A ? (v ? B ? v ? C)
  • lt Distribution of ? over ? (1) gt
  • (v ? A ? v ? B) ? (v ? A ? v ? C)
  • lt definition of ? (3) twicegt
  • (v ? A ? B) ? (v ? A ? C)
  • lt definition of ? (4) gt
  • v ? (A ? B) ? (A ? C)

55
DeMorgans First Law for sets not(A ? B)
not(A) ? not(B)
A ? B
not(A ? B)
A
A
B
B
not(A)
not(B)
A
A
B
B
56
DeMorgans First Law for sets not(A ? B)
not(A) ? not(B)
not(B)
not(A)
A
A
B
B
not(A) ? not(B)
A
B
57
Example Proof
  • Prove (S - T) ? S ? T
  • x ? (S T)
  • lt Set difference 11.22 gt
  • x ? S ? x ? T
  • lt using x ? (S) ? x ? S gt
  • x ? S ? x ? T
  • lt Set Intersection 11.21gt
  • x ? (S ? T)

58
Set Comprehension Example
  • The set of positive integers that are less than 5
  • Solution
  • x? 0 ? x ? 4x or x? 0 ? x lt5x
  • Or
  • 0,1,2,3,4

59
Set Comprehension Example
  • The set of positive integers which are divisible
    by 3 and less than 7.
  • Solution
  • x? 0ltxlt7 ? x/3x?3
  • where / is ordinary division, ? is integer
    division
  • All powers of 2.
  • Solution i? 0?i2i
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