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1 3.1

Admission in India 2015

By:

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An introduction to An introduction to Boolean AlgebrasBoolean AlgebrasAn introduction to An introduction to Boolean AlgebrasBoolean Algebras

Paolo PRINETTOPolitecnico di Torino (Italy)

University of Illinois at Chicago, IL (USA)

Paolo.Prinetto@polito.it Prinetto@uic.edu

www.testgroup.polito.it

Lecture

3.1

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3 3.1

Goal

• This lecture first provides several definitions of Boolean Algebras, and then focuses on some significant theorems and properties.

• It eventually introduces Boolean Expressions and Boolean Functions.

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4 3.1

Prerequisites

• Students are assumed to be familiar with the fundamental concepts of:

− Algebras, as presented, for instance, in:

⋅ F.M. Brown: “Boolean reasoning: the logic of boolean equations,” Kluwer Academic Publisher, Boston MA (USA), 1990, (chapter 1, pp. 1-21)

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5 3.1

Prerequisites (cont’d)

− Number systems and codes, as presented, for instance, in:

⋅ E.J.McCluskey: “Logic design principles with emphasis on testable semicustom circuits”, Prentice-Hall, Englewood Cliffs, NJ, USA, 1986, (chapter 1, pp. 1-28)

or

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6 3.1

Prerequisites (cont’d)

⋅ [Haye_94] chapter 2, pp. 51-123

or

⋅ M. Mezzalama, N. Montefusco, P. Prinetto:“Aritmetica degli elaboratori e codifica dell’informazione”,UTET, Torino (Italy), 1989 (in Italian), (chapter 1, pp. 1-38).

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7 3.1

Homework

• Prove some of the properties of Boolean Algebras, presented in slides 39 ÷ 59.

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8 3.1

Further readings

• Students interested in a deeper knowledge of the arguments covered in this lecture can refer, for instance, to:

− F.M. Brown: “Boolean reasoning: the logic of boolean equations,” Kluwer Academic Publisher, Boston MA (USA), 1990, (chapter 2, pp. 23-69 )

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9 3.1

OutlineOutline

• Boolean Algebras Definitions

• Examples of Boolean Algebras

• Boolean Algebras properties

• Boolean Expressions

• Boolean Functions.

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10 3.1

Boolean Algebras Definitions Boolean Algebras Definitions

Boolean Algebras are defined, in the literature, in many different ways:

• definition by lattices

• definition by properties

• definition by postulates [Huntington].

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11 3.1

Definition by lattices

A Boolean Algebra is a complemented distributive lattice.

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12 3.1

Definition through properties

A Boolean Algebra is an algebraic system

( B , + , · , 0 , 1 )

where:

• B is a set, called the carrier

• + and · are binary operations on B

• 0 and 1 are distinct members of B

which has the following properties:

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13 3.1

P1: idempotent

∀ a ∈ B:

• a + a = a

• a · a = a

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14 3.1

P2: commutative

∀ a, b ∈ B:

• a + b = b + a

• a · b = b · a

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15 3.1

P3: associative

∀ a, b, c ∈ B:

• a + (b + c) = (a + b) + c = a + b + c

• a · (b · c) = (a · b) · c = a · b · c

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16 3.1

P4: absorptive

∀ a, b ∈ B:

• a + (a · b) = a

• a · (a + b) = a

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17 3.1

P5: distributive

Each operation distributes w.r.t. the other one:

a · (b + c) = a · b + a · c

a + b · c = (a + b) · (a + c)

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18 3.1

P6: existence of the complement

• ∀ a ∈ B, ∃ a’ ∈ B |

− a + a’ = 1

− a · a’ = 0.

The element a’ is referred to as complement of a.

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19 3.1

Definition by postulates

A Boolean Algebra is an algebraic system

( B , + , · , 0 , 1 )

where:

• B is a set

• + and · are binary operations in B

• 0 and 1 are distinct elements in B

satisfying the following postulates:

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20 3.1

A1: closure

∀ a, b ∈ B:

• a + b ∈ B• a · b ∈ B

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21 3.1

A2 : commutative

∀ a, b ∈ B:

• a + b = b + a

• a · b = b · a

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22 3.1

A3: distributive

∀ a, b, c ∈ B:

• a · (b + c) = a · b + a · c

• a + b · c = (a + b) · (a + c)

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23 3.1

A4: identities

∃ 0 ∈ B | ∀ a ∈ B, a + 0 = a

∃ 1 ∈ B | ∀ a ∈ B, a · 1 = a

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24 3.1

A5: existence of the complement

∀ a ∈ B, ∃ a’ ∈ B |

• a + a’ = 1

• a · a’ = 0.

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25 3.1

Some definitions

• The elements of the carrier set B={0,1} are called constants

• All the symbols that get values ∈ B are called variables (hereinafter they will be referred to as x1, x2, … , xn )

• A letter is a constant or a variable

• A literal is a letter or its complement.

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26 3.1

OutlineOutline

• Boolean Algebras Definitions

⇒ Examples of Boolean Algebras

• Boolean Algebras properties

• Boolean Expressions

• Boolean Functions.

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27 3.1

Examples of Boolean Algebras

Examples of Boolean Algebras

Let us consider some examples of Boolean Algebras:

• the algebra of classes

• propositional algebra

• arithmetic Boolean Algebras

• binary Boolean Algebra

• quaternary Boolean Algebra.

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28 3.1

The algebra of classes

Suppose that every set of interest is a subset of a fixed nonempty set S.

We call

• S a universal set

• its subsets the classes of S.

The algebra of classes consists of the set 2S (the set of subsets of S) together with two operations on 2S , namely union and intersection.

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29 3.1

This algebra satisfies the postulates for a Boolean Algebra, provided the substitutions:

B ↔2S

+ ↔ ∪· ↔ ∩0 ↔ ∅1 ↔S

Thus, the algebraic system

( 2S, ∪ , ∩ , ∅ , S )

ia a Boolean Algebra.

The algebra of classes (cont'd)

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30 3.1

PropositionsPropositions

A proposition is a formula which is necessarily TRUE or FALSE (principle of the excluded third), but cannot be both (principle of no contradiction).

As a consequence, Russell's paradox :

“this sentence is false”

is not a proposition, since if it is assumed to be TRUE its content implies that is is FALSE, and vice-versa.

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31 3.1

Propositional calculus

Let:

P a set of propositional functions

F the formula which is always false (contradiction)

T the formula which is always true (tautology)

∨ the disjunction (or)

∧ the conjunction (and)

¬ the negation (not)

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32 3.1

The system

( P, ∨ , ∧ , F , T )

is a Boolean Algebra:

• B ↔ P• + ↔ ∨• · ↔ ∧• 0 ↔ F• 1 ↔ T

Propositional calculus (cont'd)

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33 3.1

Arithmetic Boolean Algebra

Let:

• n be the result of a product of the elements of a set of prime numbers

• D the set of all the dividers of n

• lcm the operation that evaluates the lowest common multiple

• GCD the operation that evaluates the Greatest Common Divisor.

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34 3.1

The algebraic system:

( D, lcm, GCD, 1, n )

Is a Boolean Algebra:

• B ↔ D• + ↔ lcm

• · ↔ GCD

• 0 ↔ 1• 1 ↔ n

Arithmetic Boolean Algebra (cont'd)

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35 3.1

Binary Boolean Algebra

The system

( {0,1} , + , · , 0 , 1 )

is a Boolean Algebra, provided that the two operations + and · be defined as follows:

+ 0 1

0 0 1

1 1 1

· 0 1

0 0 0

1 0 1

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36 3.1

Quaternary Boolean Algebra

The system

( {a,b,0,1} , + , · , 0 , 1 )

is a Boolean Algebra provided that the two operations + and · be defined as follows:

+ 0 a b 1 · 0 a b 1

0 0 a b 1 0 0 0 0 0

a a a 1 1 a 0 a 0 a

b b 1 b 1 b 0 0 b b

1 1 1 1 1 1 0 a b 1admission.edhole.com

37 3.1

OutlineOutline

• Boolean Algebras Definitions

• Examples of Boolean Algebras

⇒ Boolean Algebras properties

• Boolean Expressions

• Boolean Functions.

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38 3.1

Boolean Algebras propertiesBoolean Algebras properties

All Boolean Algebras satisfy interesting properties.

In the following we focus on some of them, particularly helpful on several applications.

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39 3.1

The Stone Representation Theorem

“Every finite Boolean Algebra is isomorphic to the Boolean Algebra of subsets of some finite set ”. [Stone, 1936]

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40 3.1

Corollary

In essence, the only relevant difference among the various Boolean Algebras is the cardinality of the carrier.

Stone’s theorem implies that the cardinality of the carrier of a Boolean Algebra must be a power of 2.

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41 3.1

Consequence

Boolean Algebras can thus be represented resorting to the most appropriate and suitable formalisms.

E.g., Venn diagrams can replace postulates.

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42 3.1

Duality

Every identity is transformed into another identity by interchanging:

• + and ·

• ≤ and ≥• the identity elements 0 and 1.

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43 3.1

Examples

a + 1 = 1

a · 0 = 0

a + a’ b = a + b

a (a’ + b) = a b

a + (b + c) = (a + b) + c = a + b + c

a · (b · c) = (a · b) · c = a · b · c

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44 3.1

The inclusion relation

On any Boolean Algebra an inclusion relation ( ≤ ) is defined as follows:

a ≤ b iff a · b’ = 0.

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45 3.1

The inclusion relation is a partial order relation, i.e., it’s:

• reflexive : a ≤ a

• antisimmetric : a ≤ b e b ≤ a ⇒ a = b

• transitive : a ≤ b e b ≤ c ⇒ a ≤ c

Properties of the inclusion relation

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46 3.1

The relation gets its name from the fact that, in the algebra of classes, it is usually represented by the symbol ⊆ :

A ⊆ B ⇔ A ∩ B’ = ∅

AABB

The inclusion relation in the algebra of classes

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47 3.1

In propositional calculus, inclusion relation corresponds to logic implication:

a ≤ b ≡ a ⇒ b

The inclusion relation in propositional calculus

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48 3.1

The following expressions are all equivalent:

• a ≤ b

• a b’ = 0

• a’ + b = 1

• b’ ≤ a’

• a + b = b

• a b = a .

Note

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49 3.1

Properties of inclusion

a ≤ a + b

a b ≤ a

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50 3.1

Complement unicity

The complement of each element is unique.

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51 3.1

(a’)’ = a

Involution

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52 3.1

(a + b)’ = a’ · b’

(a · b)’ = a’ + b’

De Morgan’s Laws

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53 3.1

Generalized Absorbing

a + a’ b = a + b

a (a’+ b) = a b

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54 3.1

Consensus Theorem

a b + a’ c + b c = a b + a’ c

(a + b) (a’ + c) (b + c) = (a + b) (a’ + c)

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55 3.1

Equality

a = b iff a’ b + a b’ = 0

Note

The formula

a’ b + a b’

appears so often in expressions that it has been given a peculiar name: exclusive-or or exor or modulo 2 sum.

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56 3.1

Boole’s expansion theorem

Every Boolean function f : Bn → B :

f (x1, x2, …, xn)

can be expressed as:

f (x1, x2, …, xn) =

= x1’ · f (0, x2, …, xn) + x1 · f (1, x2, …, xn)

∀ (x1, x2, …, xn) ∈ B

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57 3.1

Dual form

f (x1, x2, …, xn) =

= [ x1’ + f (0, x2, …, xn) ] · [x1 + f (1, x2, …, xn) ]

∀ (x1, x2, …, xn) ∈ B

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58 3.1

Remark

The expansion theorem, first proved by Boole in 1954, is mostly known as Shannon Expansion.

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59 3.1

Note

According to Stone’s theorem, Boole’s theorem holds independently from the cardinality of the carrier B.

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60 3.1

Cancellation rule

The so called cancellation rule, valid in usual arithmetic algebras, cannot be applied to Boolean algebras.

This means, for instance, that from the expression:

x + y = x + z

you cannot deduce that

y = z.

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61 3.1

Demonstration

x y z x+y x+z x+y = x+z y=z

0 0 0

0 0 1

0 1 0

0 1 1

1 0 0

1 0 1

1 1 0

1 1 1

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62 3.1

Demonstration

x y z x+y x+z x+y = x+z y=z

0 0 0 0 0 T T

0 0 1 0 1 F F

0 1 0 1 0 F F

0 1 1 1 1 T T

1 0 0 1 1 T T

1 0 1 1 1 TT FF

1 1 0 1 1 TT FF

1 1 1 1 1 T T

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63 3.1

Demonstration

x y z x+y x+z x+y = x+z y=z

0 0 0 0

0 0 1 0

0 1 0 1

0 1 1 1

1 0 0 1

1 0 1 1

1 1 0 1

1 1 1 1

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64 3.1

Demonstration

x y z x+y x+z x+y = x+z y=z

0 0 0 0 0

0 0 1 0 1

0 1 0 1 0

0 1 1 1 1

1 0 0 1 1

1 0 1 1 1

1 1 0 1 1

1 1 1 1 1

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65 3.1

Demonstration

x y z x+y x+z x+y = x+z y=z

0 0 0 0 0 T T

0 0 1 0 1 F F

0 1 0 1 0 F F

0 1 1 1 1 T T

1 0 0 1 1 T T

1 0 1 1 1 TT FF

1 1 0 1 1 TT FF

1 1 1 1 1 T T

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66 3.1

Demonstration

x y z x+y x+z x+y = x+z y=z

0 0 0 0 0 T T

0 0 1 0 1 F F

0 1 0 1 0 F F

0 1 1 1 1 T T

1 0 0 1 1 T T

1 0 1 1 1 TT FF

1 1 0 1 1 TT FF

1 1 1 1 1 T T

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67 3.1

Some Boolean Algebras satisfy some peculiar specific properties not satisfied by other Boolean Algebras.

An example

The properties:

x + y = 1 iff x = 1 or y = 1

x · y = 0 iff x = 0 or y = 0

hold for the binary Boolean Algebra (see slide #28), only.

Specific properties

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68 3.1

OutlineOutline

• Boolean Algebras Definitions

• Examples of Boolean Algebras

• Boolean Algebras properties

⇒ Boolean Expressions

• Boolean Functions.

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69 3.1

Boolean Expressions

Given a Boolean Algebra defined on a carrier B, the set of Boolean expressions can be defined specifying:

• A set of operators

• A syntax.

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70 3.1

Boolean Expressions

A Boolean expression is a formula defined on constants and Boolean variables, whose semantic is still a Boolean value.

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71 3.1

Syntax

Two syntaxes are mostly adopted:

• Infix notationInfix notation

• Prefix notation.Prefix notation.

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72 3.1

Infix notation

• elements of B are expressions

• symbols x1, x2, …, xn are expressions

• if g and h are expressions, then:

− (g) + (h)

− (g) · (h)

− (g)’

are expressions as well

• a string is an expression iff it can be derived by recursively applying the above rules.

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73 3.1

Syntactic conventions

Conventionally we are used to omit most of the parenthesis, assuming the “·” operation have a higher priority over the “+” one.

When no ambiguity is possible, the “·” symbol is omitted as well.

As a consequence, for instance, the expression

((a) · (b)) + (c)

Is usually written as:

a b + c

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74 3.1

Prefix notation

Expressions are represented by functions composition.

Examples:

U = · (x, y)

F = + (· ( x, ‘ ( y ) ), · ( ‘ ( x ), y ) )

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75 3.1

OutlineOutline

• Boolean Algebras Definitions

• Examples of Boolean Algebras

• Boolean Algebras properties

• Boolean Expressions

⇒ Boolean Functions.

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76 3.1

Boolean functions

Several definitions are possible.

We are going to see two of them:

• Analytical definition

• Recursive definition.

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77 3.1

Boolean functions:Analytical definition

A Boolean function of n variables is a function f : Bn → B which associates each set of valuesx1, x2, …, xn ∈ B with a value b ∈ B:

f ( x1, x2, …, xn ) = b.

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78 3.1

Boolean functions:Recursive definition

An n-variable function f : Bn → B is defined recursively by the following set of rules:

1 ∀ b ∈ B, the constant function defined as

f( x1, x2, …, xn ) = b, ∀ ( x1, x2, …, xn ) ∈ Bn

is an n-variable Boolean function

2 ∀ xi ∈ { x1, x2, …, xn } the projection function, defined as

f( x1, x2, …, xn ) = xi ∀ ( x1, x2, …, xn ) ∈ Bn

is an n-variable Boolean functionadmission.edhole.com

79 3.1

Boolean functions:Recursive definition (cont’d)

3 If g and h are n-variable Boolean functions, then the functions g + h, g · h, e g’, defined as

− (g + h) (x1, x2, …, xn ) =

g(x1, x2, …, xn ) + h(x1, x2, …, xn )

− (g · h) (x1, x2, …, xn ) =

g(x1, x2, …, xn ) · h(x1, x2, …, xn )

− (g’) (x1, x2, …, xn ) = (g(x1, x2, …, xn ))’

∀ xi ∈ { x1, x2, …, xn } are also n-variable Boolean functionadmission.edhole.com

80 3.1

Boolean functions:Recursive definition (cont’d)

4 Nothing is an n-variable Boolean function unless its being so follows from finitely many applications of rules 1, 2, and 3 above.

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