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Homework 9: Pattern Matching and Diagonalization


1. Pattern Matching [Online] (15 points)
Use the method given in class to design a DFA to determine all occurrences of the string 11011011001 in strings
over the alphabet f0; 1g.
You must submit and check your answers to this question using
https://grinch.cs.washington.edu/cse311/fsm.
2. Diagonalization (20 points)
Let B be the set of all infinite binary sequences that are 1 in odd positions, i.e., any string in B is of the form
1 ∗ 1 ∗ 1 ∗ 1 ∗ : : :
where we can have 0 or a 1 instead of each ∗. Show that B is uncountable using a proof by diagonalization.
3. Countability (20 points)
Complex numbers can be written as a + bi where a; b are real numbers and i is the square root of −1. Show
that subset R of complex numbers given by
R = fa + bi : a; b are rationalg
is countable
4. Irregularity (30 points)
Using the method shown in class prove that that the following languages are not regular.
(a) [15 Points] The set of binary strings of the form f0n1m0n : m < ng.
(b) [15 Points] The set of strings 0n where n is a perfect square, i.e., n = k2 for some k 2 N.
5. Undecidability (15 points)
Consider the set
Prime = f(CODE(P); x) : P reads x and halts if and only if x is a primeg
Show that Prime is undecidable using the fact that the Halting Problem is undecidable.
1

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