Propositions

In propositional logic,

• Proposition is a declarative statement declaring some fact.
• It is either true or false but not both.

 

Propositions Examples-

 

The examples of propositions are-

• 7 + 4 = 10
• Apples are black.
• Narendra Modi is president of India.
• Two and two makes 5.
• 2016 will be the lead year.
• Delhi is in India.

 

Here,

• All these statements are propositions.
• This is because they are either true or false but not both.

 

Types Of Propositions-

 

In propositional logic, there are two types of propositions-

 

 

1. Atomic propositions
2. Compound propositions

 

1. Atomic Propositions-

 

• Atomic propositions are those propositions that can not be divided further.
• Small letters like p, q, r, s etc are used to represent atomic propositions.

 

Examples-

 

The examples of atomic propositions are-

• p : Sun rises in the east.
• q : Sun sets in the west.
• r : Apples are red.
• s : Grapes are green.

 

2. Compound Propositions-

 

• Compound propositions are those propositions that are formed by combining one or more atomic propositions using connectives.
• In other words, compound propositions are those propositions that contain some connective.
• Capital letters like P, Q, R, S etc are used to represent compound propositions.

 

Examples-

 

• P : Sun rises in the east and Sun sets in the west.
• Q : Apples are red and Grapes are green.

 

Statements That Are Not Propositions-

 

Following kinds of statements are not propositions-

1. Command
2. Question
3. Exclamation
4. Inconsistent
5. Predicate or Proposition Function

 

Examples-

 

Following statements are not propositions-

• Close the door. (Command)
• Do you speak French? (Question)
• What a beautiful picture! (Exclamation)
• I always tell lie. (Inconsistent)
• P(x) : x + 3 = 5 (Predicate)

 

PRACTICE PROBLEMS BASED ON PROPOSITIONS-

 

Identify which of the following statements are propositions-

1. France is a country.
2. 2020 will be a leap year.
3. Sun rises in the west.
4. P(x) : x + 6 = 7
5. P(5) : 5 + 6 = 2
6. Apples are oranges.
7. Grapes are black.
8. Two and two makes 4.
9. x > 10
10. Open the door.
11. Are you tired?
12. What a bright sunny day!
13. Mumbai is in India.
14. I always tell truth.
15. I always tell lie.
16. Do not go there.
17. This sentence is true.
18. This sentence is false.
19. It will rain tomorrow.
20. Fan is rotating.

 

Solutions-

 

1. Proposition (True)
2. Proposition (True)
3. Proposition (False)
4. Not a proposition (Predicate)
5. Proposition (False)
6. Proposition (False)
7. Proposition (False)
8. Proposition (True)
9. Not a proposition (Predicate)
10. Not a proposition (Command)
11. Not a proposition (Question)
12. Not a proposition (Exclamation)
13. Proposition (True)
14. Proposition (True)
15. Not a proposition (Inconsistent)
16. Not a proposition (Command)
17. Proposition (True)
18. Not a proposition (Inconsistent)
19. Proposition (Will be confirmed tomorrow whether true or false)
20. Proposition (True if fan is rotating otherwise false)

Converting English Sentences To Propositional Logic-

 

While solving questions, the following replacements are very useful-

 

Word	Replacement
And	Conjunction (∧)
Or	Disjunction (∨)
But	And
Whenever	If
When	If
Either p or q	p or q
Neither p nor q	Not p and Not q
p unless q	∼q → p
p is necessary but not sufficient for q	(q → p) and ∼(p → q)

 

To understand better, let us try solving the following problems.

 

PRACTICE PROBLEMS BASED ON CONVERTING ENGLISH SENTENCES-

 

Problem-01:

 

Write the following English sentences in symbolic form-

1. If it rains, then I will stay at home.
2. If I will go to Australia, then I will earn more money.
3. He is poor but honest.
4. If a = b and b = c then a = c.
5. Neither it is hot nor cold today.
6. He goes to play a match if and only if it does not rain.
7. Birds fly if and only if sky is clear.
8. I will go only if he stays.
9. I will go if he stays.
10. It is false that he is poor but not honest.
11. It is false that he is poor or clever but not honest.
12. It is hot or else it is both cold and cloudy.
13. I will not go to class unless you come.
14. We will leave whenever he comes.
15. Either today is Sunday or Monday.
16. You will qualify GATE only if you work hard.
17. Presence of cycle in a single instance RAG is a necessary and sufficient condition for deadlock.
18. Presence of cycle in a multi instance RAG is a necessary but not sufficient condition for deadlock.
19. I will dance only if you sing.
20. Neither the red nor the green is available in size 5.

 

Solution-

 

Part-01:

 

We have-

• The given sentence is- “If it rains, then I will stay at home.”
• This sentence is of the form- “If p then q”.

 

So, the symbolic form is p → q where-

p : It rains

q : I will stay at home

 

Part-02:

 

We have-

• The given sentence is- “If I will go to Australia, then I will earn more money.”
• This sentence is of the form- “If p then q”.

 

So, the symbolic form is p → q where-

p : I will go to Australia

q : I will earn more money

 

Part-03:

 

We have-

• The given sentence is- “He is poor but honest.”
• We can replace “but” with “and”.
• Then, the sentence is- “He is poor and honest.”

 

So, the symbolic form is p ∧ q where-

p : He is poor

q : He is honest

 

Part-04:

 

We have-

• The given sentence is- “If a = b and b = c then a = c.”
• This sentence is of the form- “If p then q”.

 

So, the symbolic form is (p ∧ q) → r where-

p : a = b

q : b = c

r : a = c

 

Part-05:

 

We have-

• The given sentence is- “Neither it is hot nor cold today.”
• This sentence is of the form- “Neither p nor q”.
• “Neither p nor q” can be re-written as “Not p and Not q”.

 

So, the symbolic form is ∼p ∧ ∼q where-

p : It is hot today

q : It is cold today

 

Part-06:

 

We have-

• The given sentence is- “He goes to play a match if and only if it does not rain.”
• This sentence is of the form- “p if and only if q”.

 

So, the symbolic form is p ↔ q where-

p : He goes to play a match

q : It does not rain

 

Part-07:

 

We have-

• The given sentence is- “Birds fly if and only if sky is clear.”
• This sentence is of the form- “p if and only if q”.

 

So, the symbolic form is p ↔ q where-

p : Birds fly

q : Sky is clear

 

Part-08:

 

We have-

• The given sentence is- “I will go only if he stays.”
• This sentence is of the form- “p only if q”.

 

So, the symbolic form is p → q where-

p : I will go

q : He stays

 

Part-09:

 

We have-

• The given sentence is- “I will go if he stays.”
• This sentence is of the form- “q if p”.

 

So, the symbolic form is p → q where-

p : He stays

q : I will go

 

Part-10:

 

We have-

• The given sentence is- “It is false that he is poor but not honest.”
• We can replace “but” with “and”.
• Then, the sentence is- “It is false that he is poor and not honest.”

 

So, the symbolic form is ∼(p ∧ ∼q) where-

p : He is poor

q : He is honest

 

Part-11:

 

We have-

• The given sentence is- “It is false that he is poor or clever but not honest.”
• We can replace “but” with “and”.
• Then, the sentence is- “It is false that he is poor or clever and not honest.”

 

So, the symbolic form is ∼((p ∨ q) ∧ ∼r) where-

p : He is poor

q : He is clever

r : He is honest

 

Part-12:

 

We have-

• The given sentence is- “It is hot or else it is both cold and cloudy.”
• It can be re-written as- “It is hot or it is both cold and cloudy.”

 

So, the symbolic form is p ∨ (q ∧ r) where-

p : It is hot

q : It is cold

r : It is cloudy

 

Part-13:

 

We have-

• The given sentence is- “I will not go to class unless you come.”
• This sentence is of the form- “p unless q”.

 

So, the symbolic form is ∼ q → p where-

p : I will go to class

q : You come

 

Part-14:

 

We have-

• The given sentence is- “We will leave whenever he comes.”
• We can replace “whenever” with “if”.
• Then, the sentence is- “We will leave if he comes.”
• This sentence is of the form- “q if p”.

 

So, the symbolic form is p → q where-

p : He comes

q : We will leave

 

Part-15:

 

We have-

• The given sentence is- “Either today is Sunday or Monday.”
• It can be re-written as- “Today is Sunday or Monday.”

 

So, the symbolic form is p ∨ q where-

p : Today is Sunday

q : Today is Monday

 

Part-16:

 

We have-

• The given sentence is- “You will qualify GATE only if you work hard.”
• This sentence is of the form- “p only if q”.

 

So, the symbolic form is p → q where-

p : You will qualify GATE

q : You work hard

 

Part-17:

 

We have-

• The given sentence is- “Presence of cycle in a single instance RAG is a necessary and sufficient condition for deadlock.”
• This sentence is of the form- “p is necessary and sufficient for q”.

 

So, the symbolic form is p ↔ q where-

p : Presence of cycle in a single instance RAG

q : Presence of deadlock

 

Part-18:

 

We have-

• The given sentence is- “Presence of cycle in a multi instance RAG is a necessary but not sufficient condition for deadlock.”
• This sentence is of the form- “p is necessary but not sufficient for q”.

 

So, the symbolic form is (q → p) ∧ ∼(p → q) where-

p : Presence of cycle in a multi instance RAG

q : Presence of deadlock

 

Part-19:

 

We have-

• The given sentence is- “I will dance only if you sing.”
• This sentence is of the form- “p only if q”.

 

So, the symbolic form is p → q where-

p : I will dance

q : You sing

 

Part-20:

 

We have-

• The given sentence is- “Neither the red nor the green is available in size 5.”
• This sentence is of the form- “Neither p nor q”.
• “Neither p nor q” can be written as “Not p and Not q”.

 

So, the symbolic form is ∼p ∧ ∼q where-

p : Red is available in size 5

q : Green is available in size 5

 

Problem-02:

 

Consider the following two statements-

S1 : Ticket is sufficient to enter movie theater.

S2 : Ticket is necessary to enter movie theater.

 

Which of the statements is/ are logically correct?

1. S1 is correct and S2 is incorrect.
2. S1 is incorrect and S2 is correct.
3. Both are correct.
4. Both are incorrect.

 

Solution-

 

Statement S1 : Ticket is Sufficient To Enter Movie Theater-

 

This statement is of the form- “p is sufficient for q” where-

p : You have a ticket

q : You can enter a movie theater

So, the symbolic form is p → q

 

For p → q to hold, its truth table must hold-

 

p (Ticket)	q (Entry)	p → q (Ticket is sufficient for entry)
F	F	T
F	T	T
T	F	F
T	T	T

 

Here,

• Row-2 states it is possible that you do not have a ticket and you can enter the theater.
• However, it is not possible to enter a movie theater without ticket.
• Row-3 states it is not possible that you have a ticket and you do not enter the theater.
• However, there might be a case possible when you have a ticket but do not enter the theater.
• So, the truth table does not hold.

 

Thus, the statement- “Ticket is sufficient for entry” is logically incorrect.

 

Statement S2 : Ticket is Necessary To Enter Movie Theater-

 

This statement is of the form- “q is necessary for p” where-

p : You can enter a movie theater

q : You have a ticket

So, the symbolic form is p → q

 

For p → q to hold, its truth table must hold-

 

p (Entry)	q (Ticket)	p → q (Ticket is necessary for entry)
F	F	T
F	T	T
T	F	F
T	T	T

 

Here, All the rows of the truth table make the correct sense.

Thus, the statement- “Ticket is necessary for entry” is logically correct.

 

Thus, Option (B) is correct.