CS Problem Set 2

NOTE: Each problem set counts 15% of your mark, and it is important to do your own
work. You may consult with others concerning the general approach for solving problems on
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DIRECTIONS: For each of the following languages A, determine whether A is semidecidable, and whether A is semidecidable. Justify your

answers.
1.
A1 = {hMi | M is a TM and M, starting with a blank tape,
eventually writes a nonblank symbol}
2. Let A2 be the set of all Turing Machine descriptions hMi such that M, during the
computation starting with a blank tape, never attempts to move its head left when its
head is on the left-most tape square. (Here we use the textbook’s convention that the
tape is one-way infinite to the right.)
3. Let A3 be the set of all hMi such that M is a Turing machine and L(M) contains
infinitely many even length palindromes. (Recall that these are strings of the form
wwR, where wR is w written backwards). Here we assume that Σ = {0, 1}.
4. A4 is the set of all hG1, G2i such that G1 and G2 are context-free grammars, and
L(G1) = L(G2).
1

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