CMPSCI 250: Introduction to Computation

Final Exam

David Mix Barrington

10 May 2011

Directions:

• Answer the problems on the exam pages.
• There are three problems for 125 total points. Actual scale is A=98, C=63.
• If you need extra space use the back of a page.
• No books, notes, calculators, or collaboration.

  Q1: 30 points
  Q2: 35 points
  Q3: 60 points
Total: 125 points

Question 1 (30): As on the first midterm, we have a set of dogs G, a set of Playthings P, constants Ace (a), Biscuit (b), Cardie (c), and Duncan (d) of type Dog, and constants Rope Chew (r), Stuffed Chicken (s), and Tennis Ball (t) of type Plaything, with possibly other elements of G and P as well. The predicate H(g, p) means "Dog g has Plaything p". We define two new predicates X and Y, each taking two Dogs as arguments. X(g, g') is defined to mean "∀p: H(g, p) → H(g', p)" and Y(g, g') is defined to mean "∀p: H(g, p) ↔ H(g', p)".

• (a, 10) Determine whether X is an equivalence relation, whether X is a partial order, whether Y is an equivalence relation, and whether Y is a partial order. Justify your answers, showing that you know the definitions of these two types of relations and the properties that define them. (You do not need to give quantifier proofs that the relations have these properties.)

• (b, 10) Assuming that X(a, b), Y(a, c), and Y(b, d) are all true, give a quantifier proof of X(c, d).

• (c, 5) Translate these two statements as indicated:

  • (I, to symbols): Either Cardie has the Rope Chew, or Duncan has the Stuffed Chicken only if Biscuit has the Stuffed Chicken, but not both.

  • (II, to English): (H(a, t) ∨ H(c, r)) → (H(d, t) ∨ H(a, r))

• (d, 5) Choose one of the statements of part (c), and prove it given the assumptions of part (b). (I guarantee only that at least one of those two statements can be proved from those assumptions.) (A prose proof is sufficient if it is convincing.)
• Question 2 (35): Let M be the following λ-NFA. The state set is {1, 2, 3, 4, 5}, the start state is 1, the final state set is {2, 5}, and the set of transitions is {(1, b, 3), (1, λ, 4), (2, λ, 5), (3, λ, 2), (4, a, 2), (4, λ, 5), (5, a, 5)}.

  • (a, 10) Build an ordinary NFA M' such that L(M') = L(M).

  • (b, 10) Build a DFA D such that L(D) = L(M').

  • (c, 10) Find a regular expression R such that L(R) = L(M). You may choose to use the state elimination construction starting with M, M', D, or even a minimized version of D.

  • (d, 5) Build a λ-NFA whose language is that denoted by the following regular expression: (b + a∅b^*)ba + aba. Carry out the construction from the book (or from lecture) without first simplifying the regular expression. (Note that this expression may have nothing to do with parts (a)-(c) of this question.)

• Question 3 (60): In the game of Scrabble^TM, each of the letters in the alphabet {A, B, ..., Z} is assigned a numerical value from the set {1, 2, ..., 10}. We define LV(ch) to be the value of the letter ch, so that for example LV('D') = LV('G') = 2 and LV('O') = 1. The function WV gives the value of a string over this alphabet, which is the sum of the values of the letters in the string, so that WV("DOG") = 2 + 1 + 2 = 5. Finally, for any fixed natural number n, we define the language C_n to be the set of all strings w such that WV(w) = n.

  • (a, 5) Give a recursive definition of the function WV for all strings w, with a base case for the empty string and an inductive case for adding a letter to a string.

  • (b, 5) Give a recursive pseudo-Java method int wv (string w) to compute WV for any pseudo-Java string, using the base methods isEmpty, last, and allButLast. (Recall that last("DOG") is 'G' and that allButLast("DOG") is "DO". Also remember that these are pseudo-Java string values, where string is a primitive type, not real Java String objects.) Don't worry about input strings that are not over the correct alphabet. You are given a method int lv (char ch) that returns LV(ch) for any character ch in that alphabet.

  • (c, 10) Prove that for any natural number n, the language C_n defined above is regular. (Hint: You can build a DFA with exactly n + 2 states.)

  • (d, 10) Let Z be the language {1ⁿ$w: w ∈ C_n} over the alphabet {1, $, A, ..., Z}. (An example of a string in Z is "11111$DOG".) Is Z a regular language? Prove your answer. Is Z the language of any two-way DFA?

  • (e, 10) Is the language Z, defined in part (d), Turing decidable? Justify your answer, making clear that you understand the definitions. Is Z Turing recognizable?

  • For the last two parts we consider the following weighted directed graph G. G has a node for every string w with WV(w) ≤ 30. If w is any string and a is any letter, and there is a node for the string wa in G, then there is an edge from w to wa, with weight LV(a). Let D be the set of nodes in G that correspond to words in the standard Scrabble dictionary.

  • (f, 10) Prove the following by strong induction for all natural numbers (non-negative integers) k: "If k ≤ 30, and w is any string with WV(w) = k, then there is a path in G from the node for λ to the node for w, and this path has total weight k."

  • (g, 10) Suppose we carry out a uniform-cost search of G, with D as the set of goal nodes, starting from the node for some particular string u. Which goal node will we find, if any?

Last modified 9 May 2012