# Second Midterm Exam Spring 2016

### Directions:

• Answer the problems on the exam pages.
• There are four problems, some with multiple parts, for 100 total points plus 10 extra credit. Actual scale A = 93, C = 66.
• Some useful definitions precede the questions below.
• No books, notes, calculators, or collaboration.
• In case of a numerical answer, an arithmetic expression like "217 - 4" need not be reduced to a single integer.

```  Q1: 20 points
Q2: 15 points
Q3: 40 points
Q4: 25+10 points
Total: 100+10 points
```

Here are definitions of some terms, sets, predicates, and statements used on this exam.

Remember that the scope of any quantifier is always to the end of the statement it is in.

Remember that a natural is a non-negative integer, so that the set N or all naturals is {0, 1, 2, 3,...}.

A binary string is a string over the alphabet {0, 1}, and the reversal wR of a binary string w is defined by the rules λR = λ and (wa)R = awR.

An undirected graph is said to be bipartite if it is possible to partition its nodes into two sets R and B, such that every edge of the graph has one endpoint in R and one in B. Equivalently, the nodes may be colored red and blue such that no edge connects red to red or blue to blue.

On a two-dimensional grid, the Manhattan distance between two points (a, b) and (c, d) is the minimum length of any path from one point to the other that follows the edges of the grid, that is, |a - c| + |b - d|.

The following scenario is used in Question 4. On a shelf in our house there are two containers of dog treats, one with large treats for Cardie and one with small treats for Duncan. Every day Cardie gets five large treats and Duncan gets seven small treats. If there are fewer than 20 large treats in Cardie's container at the beginning of the day, we add 48 more. If there are fewer than 20 small treats in Duncan's container at the beginning of the day, we add 90 more. (For example, suppose there are 23 treats in Cardie's container at the start of Day 0. Then there are 23 - 5 = 18 there at the start of Day 1, and 18 + 48 - 5 = 61 at the start of Day 2.

The function c(n) denotes the number of treats in Cardie's container at the start of Day n, and the function d(n) denotes the number of treats in Duncan's container at the start of Day n.

The n by n knight graph has an n by n square array of nodes, labeled as in chess positions with rows 1, 2,..., n and columns a, b,..., so the for example node d3 is in the third row and the fourth column. There is an undirected edge between two nodes whenever a chess knight can move from one to the other. (Chess knights move either two squares horizontally and one vertically, or one horizontally and two vertically.

This page shows pictures of the 3 by 3, 4 by 4, 5 by 5, and 6 by 6 knight graphs. (Note that neither the 1 by 1 nor the 2 by 2 knight graphs have any edges at all.) The 3 by 3 knight graph has 8 edges, and the 4 by 4 knight graph has 24.

Thus if f(n) is the number of edges in the n by n knight graph, we have all the values for n ≤ 4. By an analysis we won't explain here, for all n with n ≥ 4, f(n+1) is equal to f(n) + 20 + 4(2n - 7). You will need this formula for Question 2.

This page has a 4 by 4 knight graph with labels, which are not exactly the labels we are using. We are calling the nodes a1, a2,..., d4, and we will normally put the a nodes in a column rather than a row, but the 4 by 4 kn1ght graph you will search in Question 3 otherwise looks the same as this one.

• Question 1 (20): The following are ten true/false questions, with no explanation needed or wanted, no partial credit for wrong answers, and no penalty for guessing. Each one counts two points.

• (a) If a rooted tree has only one node, its depth is 1.

• (b) Define a binary predicate X(i, j) on the naturals as follows. X(0, 0) is false, x(0, k) is true whenever k ≠ 0, and X(i+1, j+1) always has the same value as X(i, j). Then for any i and j, it must be true that X(i, j) is true if and only if i ≠ j.

• (c) If an undirected graph with n nodes has fewer than n edges, it cannot have a cycle.

• (d) For any binary strings u and v, (uRvR)R = uv.

• (e) A generic search may continue after a goal node has been put on its open list.

• (f) Let G be a two-player deterministic game with perfect information and a finite game tree with real-number values at its leaves. It is possible that G has value x, but that none of the leaves of the tree has value x.

• (g) Let G be a game as in part (f), where White (who wants the game result to be as large as possible) has the first move, and every leaf in the left subtree of the root node has value at least x. Then the value of the game is at least x.

• (h) There exists a rooted binary tree T, with a positive even number of nodes, such that every internal node of T has exactly two children. (Recall that a node of such a tree is called an internal node if and only if it is not a leaf.

• (i) Let P(w) be a predicate with one free variable w of type "binary string". It is possible that P(λ) is true and that P(w) implies ((P(w0) ∧ P(w1)) for any string w, but that there exists a string v sich that P(v) is false.

• (j) For every positive natural n, there is an arithmetic expression En such that (1) the only operators of En are + and ×, (2) the only atomic values of En are prime numbers, and (3) En evaluates to n. (Note that the empty string λ is not an arithmetic expression.)

• Question 2 (15): Let f(n) be the number of edges in the n by n knight graph, as defined above. Prove that for all naturals n with n ≥ 1, f(n) = 4(n - 1)(n - 2). (Hint: Check enough base cases until you can use the formula given above for your inductive step.)

• Question 3 (40): These questions all involve searches of the 4 by 4 knight graph, defined above. Its sixteen nodes are labeled a1, a2,..., d3, d4.

• (a, 5) Carry out a depth-first search of the graph with start node a1 and goal node c3. Assume that once a node has been put on the open list, it is recognized and not put on the open list again.

• (b, 5) Carry out a breadth-first search of the graph with start node a1 and goal node c3. As in part (a), assume that once a node has ben put on the open list, it is recognized and not put on the open list again.

• (c, 15) Carry out another breadth-first search of the graph with start node a1, but this time with no goal node. (You may refer to your work in part (b) and use it without copying it.) Redraw the graph as a BFS tree, indicating both tree and non-tree edges.

• (d, 5) Recall the definition of a bipartite graph given above. Is the 4 by 4 knight graph bipartite? How can you tell that it is our isn't, given the BFS tree from part (c)?

• (e, 10) Label the edges of the 4 by 4 knight graph such that edges that go two steps vertically, such as from a1 to b3, are labeled 3, and edges that go two steps horizontally, such as from a1 to c2, are labeled 5. Conduct a uniform-cost search of the weighted graph, with start node a1 and goal node c3.

• (f, 5) Suppose we want to convert our uniform-cost search of the labeled graph of part (e) into an A* search. Would Manhattan distance (as defined above) from c3 be a suitable heuristic? Why or why not?

• Question 4 (25+10): These questions involve the dog treat scenario defined above.

• (a, 5) As above, let c(n) and d(n) be the number of treats in Cardie's and Duncan's containers, respectively, at the start of Day n. Write down rules explaining how c(n+1) and d(n+1) depend on c(n) and d(n), respectively. (Hint: Use a definition by cases or Java `if` statements.)

• (b, 5) Assume that c(0) = 34 and that d(0) = 46. Find the values of c(7) and d(7).

• (c, 15) With the assumptions of part (b), prove by induction that for every natural n, c(n) + d(n) (the total number of treats in the two containers at the start of Day n) is not divisible by 6. (Hint: Use ordinary induction, referring to the rules in part (a) for your inductive step.

• (d, 10XC) Using the assumptions of parts (b) and (c), prove the following statement:

∃x: ∀y: ∃n: (n > y) ∧ (c(n) + d(n) = x)

(Hint: One way to do this is by contradiction. Your proof may be informal if it is convincing.)