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Overview of Mathematical Logic

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Estimated time: 18 minutes
Maharashtra State Board: Class 12

Definition: Statement

A statement is a declarative (assertive) sentence which is either true or false, but not both simultaneously. Statements are denoted by p, q, r, ....

Open Statement:

An open sentence is a sentence whose truth can vary depending on conditions not stated in the sentence.

Maharashtra State Board: Class 12

Definition: Truth Value of a Statement

Each statement is either true or false. If a statement is true, then its truth value is 'T', and if the statement is false, then its truth value is F.

Maharashtra State Board: Class 12

Definition: Logical Connectives

The words or phrases which are used to connect two statements are called logical connectives.

Eg. 'and', 'or', 'if ..... then', 'if and only if ', 'not''.

Maharashtra State Board: Class 12

Definition: Simple and Compound Statements

A statement which cannot be split further into two or more statements is called a simple statement. If a statement is the combination of two or more simple statements, then it is called a compound statement.

Maharashtra State Board: Class 12

Key Points: Logical Connectives

Connective Symbol Name True When
and Conjunction Both true
or Disjunction At least one true
if...then Conditional False only when T → F
iff Biconditional Same truth values
not ~ Negation Opposite value

Note:  ~ (~ p) =   p

Maharashtra State Board: Class 12

Definition: Logical Equivalence

Two statement patterns are said to be equivalent if their truth tables are identical. If statement patterns A and B are equivalent, we write it as A ≡ B.

Maharashtra State Board: Class 12

Key Points: Tautology, Contradiction and Contingency

Type Meaning
Tautology Always True
Contradiction Always False
Contingency Sometimes T, Sometimes F
Maharashtra State Board: Class 12

Key Points: Quantifiers and Quantified Statements

Symbol  Meaning
For all (Universal quantifier)
There exists (Existential quantifier)

Quantified statement: An open sentence with a quantifier becomes a statement and is called a quantified statement.

Maharashtra State Board: Class 12

Key Points: Negation of Compound Statements

Type Given Statement Negation Symbolic Form
Negation of Conjunction p ∧ q Not p or Not q ~(p ∧ q) ≡ ~p ∨ ~q
Negation of Disjunction p ∨ q Not p and Not q ~(p ∨ q) ≡ ~p ∧ ~q
Negation of Implication p → q p and Not q ~(p → q) ≡ p ∧ ~q
Negation of Biconditional p ↔ q (p and Not q) or (q and Not p) ~(p ↔ q) ≡ (p ∧ ~q) ∨ (q ∧ ~p)
Negation of Quantified Statement ∀ x P(x) / ∃ x P(x) Replace “all” by “some” and vice versa, and negate P(x) ~(∀ x P(x)) ≡ ∃x ~P(x) 
~(∃x P(x)) ≡ ∀x ~P(x)
Maharashtra State Board: Class 12

Key Points: Algebra of Statements

Sr. No. Law Name Logical Form
1 Idempotent Law p ∧ p ≡ p
p ∨ p ≡ p
2 Commutative Law p ∧ q ≡ q ∧ p 
p ∨ q ≡ q ∨ p
3 Associative Law p ∧ (q ∧ r) ≡ (p ∧ q) ∧ r ≡ p ∧ q ∧ r 
p ∨ (q ∨ r) ≡ (p ∨ q) ∨ r ≡ p ∨ q ∨ r
4 Distributive Law p ∧ (q ∨ r) ≡ (p ∧ q) ∨ (p ∧ r)
p ∨ (q ∧ r) ≡ (p ∨ q) ∧ (p ∨ r)
5 De Morgan’s Laws ~(p ∧ q) ≡ ~p ∨ ~q
~(p ∨ q) ≡ ~p ∧ ~q
6 Identity Laws p ∧ T ≡ p
p ∨ F ≡ p
p ∧ F ≡ F
p ∨ T ≡ T
7 Complement Laws p ∧ ~p ≡ F
p ∨ ~p ≡ T
8 Absorption Laws p ∨ (p ∧ q) ≡ p
p ∧ (p ∨ q) ≡ p
9

Conditional Law

p → q ≡ ~p ∨ q

10 Biconditional Law

p ↔ q ≡ (p → q) ∧ (q → p) ≡ (~p ∨ q) ∧ (~q ∨ p)

Maharashtra State Board: Class 12

Key Points: Switching Circuits

Circuit Type Logical Form
Series p ∧ q
Parallel p ∨ q

Switch ON = 1
Switch OFF = 0

Maharashtra State Board: Class 12

Key Points: Converse, Inverse, Contrapositive

For p → q:

Type Form
Converse q → p
Inverse ∼p → ∼q
Contrapositive ∼q → ∼p
Maharashtra State Board: Class 12

Definition: Duality

Two compound statements s1 and s2 are said to be duals of each other if one can be obtained from the other by:

  • Replacing ∧ (and) by ∨ (or)

  • Replacing ∨ (or) by ∧ (and)

  • Replacing T (tautology) by F (contradiction)

  • Replacing F (contradiction) by T (tautology)

while keeping negations unchanged.

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