CS130 Mathematics for Computer Scientists 1
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CS130 Mathematics for Computer Scientists 1
Problem set 4
Each of the ten problems is worth 10 points, for a total of 100. You must justify all your answers.
1. Consider two functions i and j that map English words of 2 or more letters to the set {a, b, . . . , z} by the following rule: the value of i is the first letter of the word, and the value of j is the second letter of the word. For example, i(example) = e and j(example) = x. Is it true that i-1 (a) \ j -1 (b) = {answer} ?
2. Let T = {a, b, c, d} and P = 2T. Consider the relation < defined on P by the following rule: A < B if and only if A n (T \ B) C {c}. Is this relation (a) reflexive, (b) symmetric, (c) antisymmetric?
3. Give a binary relation R on an n-element set such that R ∅ and IR · R-1 I = n2 o IR-1 · RI.
4. A tree contains, for each d e {3, 9, 10, 11, 12}, exactly one vertex of degree d. All other vertices have degrees 1 and 2. How many vertices have degree 1?
5. (a) Prove that if a directed graph G has a cycle, then this G cannot be a Hasse diagram of any partial order.
(b) Find a directed graph that has 5 or fewer vertices, has no loops, parallel edges, or cycles,
and is not a Hasse diagram of any partial order.
6. Find a logical flaw in the following ‘proof’ of the claim that every connected undirected graph G = (V, E) with IV I = IEI + 1 is acyclic:
“Induction on IV I. Base case: if IV I = 1, then G has a single vertex and no edges, so the statement holds. Inductive step: let us assume the claim holds for every graph G = (V, E) on n vertices. We prove that it also holds for a graph G/ = (V/ , E/ ) that arises from G by adding a new vertex. In order for the assumption IV/ I = IE/ I + 1 to hold for G/ , we also add one new edge. Since G/ is assumed to be connected, this edge must connect the new vertex to some vertex in V . Therefore, the new vertex has degree 1 and so it cannot belong to a cycle. The inductive hypothesis already ensures that G has no cycle, so in fact neither has G/ .”
7. If x1 , x2 , . . . , xn e Q, prove that Ix1 I + Ix2 I + . . . + IxnI > Ix1 + x2 + . . . + xnI.
8. Prove that the set M = {x e R : x2 = 2a 3b, a e N, b e N} is countably infinite. Your solution should either give a bijective function f : N 二 M or give pseudo-code of a program that prints all the values of such a function, in the following format (in this case you don’t have to define f explicitly, but need to also show the first five lines of output):
0: element of M
1: another element of M
2: yet another element of M
. . .
9. Let X be any set. Let 0 denote, as usual, the number zero. Prove that X \ {0} is countable if and only if X is countable. (If needed: use facts/conventions from our lectures only.)
10. Denote, for every finite set A C Z, by s(A) the number I{(x, y, z) e A3 : 5x _ 6y + 7z = 0}I.
(a) Is it true or false that s(A) < IAI2 for every finite A C Z ?
(b) Give a nonempty finite set A C Z such that s(A) = IAI2 or prove that no such A exists.
2021-12-08