CS402 Assignment 1 Solution and Discussion

Assignment No. 01
Semester: Fall 2019
Theory of Automata – CS402
Total Marks: 20Due Date: November 15, 2019
Objectives:
Objective of this assignment is to assess the understanding of students about the concept of languages, regular expressions and finite automata.Instructions:
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Question: Marks = 20
Q1. Show that the following pairs of regular expressions define the same language over the alphabetL = {a, b}.
(i) (ab)a and a(ba)
(ii) (a* + b)* and (a + b)*
(iii) (a* + b*)* and (a + b)*
[9 marks = 3*3]
Q2. Develop a regular expression for the following language over the alphabet P = {a, b} such that it accepts all strings in which the letter b is never tripled. This means that no word contains the substring bbb. [5 marks]Q3. Develop a regular expression for the following language over the alphabet P = {a, b} such that it accepts all strings all words in which a is tripled or b is tripled, but not both. This means each word contains the substring aaa or the substring bbb but not both. [ 6 marks]
BEST OF LUCK

Q1. Show that the following pairs of regular expressions define the same language over the alphabet L = {a, b}.
(i) (ab)a and (ii) (a + b)* and (iii) (a* + b*)* and
a(ba)* (a + b)* (a + b)*
Solution:
(i) (ab)a and a(ba) represents same language.
Both will never generate bb’s and both are starting and ending with a
(ii) (a* +b)* (a+b)*
[9 marks = 33]
(a)+ b a* + b*
a* + b* a* + b*
Both are all strings possible in L = {a,b}
(iii) (a* +b*)* (a+b)* (a*)* + (b*)*
a* + b*
(a + b)* Language with all strings
Q2. Develop a regular expression for the following language over the alphabet P = {a, b} such that it accepts all strings in which the letter b is never tripled. This means that no word contains the substring bbb. [5 marks]
Solution:
The required R.E will be (˄+b+bb) (a+ab+abb)*Q3. Develop a regular expression for the following language over the alphabet P = {a, b} such that it accepts all strings all words in which a is tripled or b is tripled, but not both. This means each word contains the substring aaa or the substring bbb but not both. [ 6 marks]
Solution:
(a+ ba + bba)* (˄ + b + bb ) aaa ( a+ ba + bba)* (bb + b + ˄) + (b+ ab + aab)* (aa + a + ˄) bbb (b + ab + aab)* (aa+ a + ˄)


Q1. Show that the following pairs of regular expressions define the same language over the alphabet L = {a, b}.
(i) (ab)a and (ii) (a + b)* and (iii) (a* + b*)* and
a(ba)* (a + b)* (a + b)*
Solution:
(i) (ab)a and a(ba) represents same language.
Both will never generate bb’s and both are starting and ending with a
(ii) (a* +b)* (a+b)*
[9 marks = 33]
(a)+ b a* + b*
a* + b* a* + b*
Both are all strings possible in L = {a,b}
(iii) (a* +b*)* (a+b)* (a*)* + (b*)*
a* + b*
(a + b)* Language with all strings
Q2. Develop a regular expression for the following language over the alphabet P = {a, b} such that it accepts all strings in which the letter b is never tripled. This means that no word contains the substring bbb. [5 marks]
Solution:
The required R.E will be (˄+b+bb) (a+ab+abb)*Q3. Develop a regular expression for the following language over the alphabet P = {a, b} such that it accepts all strings all words in which a is tripled or b is tripled, but not both. This means each word contains the substring aaa or the substring bbb but not both. [ 6 marks]
Solution:
(a+ ba + bba)* (˄ + b + bb ) aaa ( a+ ba + bba)* (bb + b + ˄) + (b+ ab + aab)* (aa + a + ˄) bbb (b + ab + aab)* (aa+ a + ˄)

