CS 251 SAMPLE EXAM QUESTIONS A

CS 251 SAMPLE EXAM QUESTIONS
P. Find truth values for the propositional variables A, B, and C such that the truth
value of the following wff is false.
(A ∨ B → C) ∧ A → (C → B).
P. Use basic equivalences to prove the following equivalence.
¬ ((¬ A ∧ B) ∨ (A ∧ ¬ B)) ≡ (¬ A ∧ ¬ B) ∨ (A ∧ B).
P. Use basic equivalences to prove that the following wff is a tautology. In other
words, show the wff is equivalent to true.
¬ B ∧ (A → B) → ¬ A.
P. Demonstrate the use of Quine’s method to find out whether the following wff is a
tautology, a contradiction, or a contingency.
(A → B) → (A ∨ C → B ∨ C).
P. Given the truth function ƒ defined by the following table.
A
true
true
false
false
B
true
false
true
false
ƒ(A, B)
true
false
false
true
a. Write ƒ(A, B) in CNF (conjunctive normal form).
b. Write ƒ(A, B) in DNF (disjunctive normal form).
P. Find each normal form for the following wff. (Full normal form is not required.)
(A → B) → (C→ D)
a. Disjunctive Normal Form (DNF)
b. Conjunctive Normal Form (CNF)
P. Give a formal proof of the following wff using the CP rule. Use only premises and
inference rules. i.e., no T’s allowed.
(A ∨ B → C) ∧ (C ∨ D → E ∧ F) → (A → F).
P. Find a conjunctive normal form (CNF) for the following wff (Full CNF is NOT
required).
(A ∨ B) → (C ∧ D).
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P. Give a formal proof that the following wff is a tautology by using the CP rule. DO
NOT use any T’s.
(A ∨ B → C) ∧ (C → D ∧ E) → (A → D).
P. Give a formal proof that the following wff is a tautology by using the CP rule. DO
NOT use any T’s..
(¬ A ∨ ¬B) ∧ (B ∨ C) ∧ (C → D) → (A → D).
P. Give a formal proof that the following wff is a tautology by using the IP rule. Use
T only for false.
(A ∨ B) ∧ (A → B) → B.
P. Give a formal proof that the following wff is a tautology by using the IP rule. Use
T twice: once to transform the negation of the consequent and once for false.
(¬ A ∨ ¬ B) ∧ (B ∨ C) ∧ (C → D) → (A → D).
P. a. (2 points). What does it mean for a proof system to be sound?
b. (2 points). What does it mean for a proof system to be complete?
c. (6 points). Write down the three Frege-Lukasiewicz Axioms.
P. Write down the proposition denoted by each of the following wffs over the domain
{a, b}.
a. ∀x ∃y p(x, y).
b. ∃x ∀y p(x, y)
P. In the following wff, label each occurrence of a variable as either B (for bound) or
F (for free).
∃x (p(x, y) ∧ ∀y (p(x, y, z) → ∃z q(x, y, z, w))).
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P. Describe a model for each of the following wffs.
a. ∃x p(x) ∧ ∃x ¬ p(x).
b. ∀x ∃y (p(x, y) ∧ ¬ p(y, x)).
P. Describe a countermodel for each of the following wffs.
a. ∃x p(x) → ∀x p(x).
b. ∃x p(x) ∧ ∃x q(x) → ∃x (p(x) ∧ q(x)).
c. ∀y ∃x p(x, y) → ∃x ∀y p(x, y).
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P. Find a countermodel for each of the following wffs.
a. ∀y ∃x p(x, y) → ∃x ∀y p(x, y).
b. ∀x (p(x) ∨ q(x)) → ∀x p(x) ∨ ∀x q(x).
P. Given the following wff.
p(a, b) ∧ ∀x ∀y (p(x, y) → q(x, y)) → ∃x ∃y q(x, y).
Prove that the wff is valid as follows: let I be an arbitrary interpretation with
domain D such that I is a model for the antecedent. Show that I is a model for the
consequent.
P. Given the following wff.
p(a, b) ∧ ∀x ∀y (¬ p(x, y) ∨ q(x, y)) ∧ ∀x ∀y ¬ q(x, y).
Prove that the wff is unsatisfiable as follows: Assume that it has a model and
find a contradiction.
P. Prove each the following equivalences by starting on one side and proceeding to
the other side using known equivalences.
a. ∀x ∃y (p(x) → q(y)) ≡ ∃y ∀x (p(x) → q(y)).
b. ∀x (p(x) → ∀y (q(y) → r(x, y))) ≡ ∀x ∀y (p(x) ∧ q(y) → r(x, y))
P. Use equivalences to construct the indicated prenex normal form for the following
wffs. (SHOW ALL WORK)
a. (prenex CNF) ∀y ∃x (p(x, y) ∧ ∃x q(x)) → ∀x (p(x, y) ∨ q(x)).
b. (prenex DNF) ∀x (∃x ∀y (q(x) ∨ r(x, y)) → p(x)).
P. Let C(x) mean x is a child, V(x) mean x is a vegetable, and L(x, y) mean x likes y.
Find a wff to formalize each of the following sentences about things.
a. Every child hates some vegetable.
b. Some child hates all vegetables.
c. Only adults like vegetables.
P. Give an example a wff W(x) and a term t such that t is not free to replace x in
W(x).
P. Write down a formal proof to show the following wff is valid. No T’s allowed.
∀x (p(x) → ∀y (q(y) → r(x, y))) → ∀x ∀y (p(x) ∧ q(y) → r(x, y))
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P. Use the CP rule to give a formal proof of the following wff. Use only premises and
inference rules. i.e., no T’s allowed.
∀x ∀y (p(x, y) → q(x, y) ∨ r(x, y)) ∧ ∃x ∃y (p(x, y) ∧ ¬ r(x, y)) → ∃x ∃y q(x, y)
P. Write down a formal proof using (IP) to show that the following wff is valid. You
may use T two times; once to transform the negation of the consequent; once for
false.
∀x ∀y (p(x, y) → q(x)) ∧ ∀x ∃y p(x, y) → ∀x q(x).
P. Write down a formal proof to show the following wff is valid. No T’s allowed.
[∃x ∀y p(x, y) ∧ ∀x (p(x, x) → ∃y q(y, x))] → ∃y ∃x q(x, y).
P. Give a formal proof to show the following wff is valid in a first-order theory with
equality. No T’s allowed.
∀x ((x ≠ ƒ(a)) ∨ (ƒ(x) ≠ t)) → (ƒ(ƒ(a)) ≠ t ).
P. Prove the correctness of the following wff where x and y are integers.
{x > y + 1} x := x – 1; y := y + 1 {x ≥ y}.
P. Prove the correctness of the following wff, where int(x) means x is an integer.
{int(x) ∧ x > 0} if x > 1 then x := x – 1 {x ≥ 1}.
P. Prove the correctness of the following wff.
{x = y + z} if y < z then z := y else y := z {x ≥ y + z}.
P. A function ƒ has a fixed point x if ƒ(x) = x. Write down a formal wff to represent
the following sentence.
“The function ƒ has exactly two fixed points.”
P. Prove the correctness of the following wff, where x and y take integer values.
{∃x (y = 2x)} y := y – 5 {∃x (y = 2x + 1)}.
P. Use the if-then-else rule to prove the correctness of the following wff, where x
and y take integer values, and odd(x) and even(x) mean that x is odd and x is
even, respectively.
{x < y} if odd(y – x) then y := y – 1 else x := x + 2 {x ≤ y}.
P. Given the following wff where int(x) means x is an integer, odd(x) means x is odd,
and even(x) means x is even.
{(x > 1) ∧ int(x)} while odd(x) do x := (x + 1)/2 od {(x > 1) ∧ int(x) ∧ even(x)}.
Use the while-rule to prove the correctness of the wff.
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P. Given the following wff where int(x) means x is an integer, odd(x) means x is odd,
and even(x) means x is even.
{(x > 1) ∧ int(x)} while odd(x) do x := (x + 1)/2 od {(x > 1) ∧ int(x) ∧ even(x)}.
Prove termination of the while-loop, where (x > 1) ∧ int(x) is the loop invariant.
P. Complete the precondition P so that the following wff matches the Array
Assignment Axiom (AAA).
{P} a[n] := t {(a[n] < b) ∨ (a[m] ≥ c)}.
P. Write down the minimal order of logic to which each wff belongs.
a.
b.
c.
d.
∀x ∀y (p(x, y) → ∃L (L(x) ∧ L(y))).
∀x ∃S ∃T (S(x) ∧ T(S, x) → U(T, S, x)).
∀x ∃ƒ (p(x) ∧ q(x) → r(ƒ(x), x)).
∀x ∃y (p(ƒ(x)) ∧ q(y) → r(x, y)).
P. Assume that a line is a set of points. Formalize the following statement as as
higher-order wff..
“For every line L and every point x not on L there is another line M passing
through x such that L and M have no points in common."
P. For each of the following wffs circle the properties that apply
a. ∀x ∃p p(x).
{valid, invalid, satisfiable, unsatisfiable}
b. ∀p ∃x p(x).
{valid, invalid, satisfiable, unsatisfiable}
c. ∃x ∀p p(x).
{valid, invalid, satisfiable, unsatisfiable}
d. ∃p ∀x p(x).
{valid, invalid, satisfiable, unsatisfiable}
P. Give a formal proof using IP to show that the following wff is valid in a first-order
theory with equality.
∃x (ƒ(ƒ(ƒ(x))) = t ) → ∃x ∃y [(x = ƒ(y)) ∧ (ƒ(ƒ(x)) = t)].
P. Prove the correctness of the following wff, where p(x) means that x is a positive
integer.
{ p(x)}
while x > 2 do x := x – 2 od
{ p(x) ∧ (x ≤ 2)}
P. Describe the general process for proving that the following loop terminates, where
P is a loop invariant.
{P}
while C do S od
{P ∧ ¬ C}
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P. Find a most general unifier for the following two atoms.
p(ƒ(x), ƒ(x), g(a))
and
p(ƒ(g(z)), w, g(z)).
P. Transform the following wff into clausal form.
∃x ∀y p(x, y) ∧ ∀x (p(x, x) → ∃y q(x, y))
P. Use resolution to prove the unsatisfiability of the five clauses that are listed as
premises.
1.
2.
3.
4.
¬ r(x) ∨ ¬ p(ƒ(a))
p(ƒ(x)) ∨ ¬ q(x)
r(w) ∨ ¬ s(ƒ(y))
s(z) ∨ ¬ p(ƒ(y))
P
P
P
P
5. q(a)
P
P. Prove that the following wff is valid by using resolution to prove that its negation
is unsatisfiable.
∃x ∀y p(x, y) → ∀y ∃x p(x, y).
P. Given the following logic program.
p(a)
p(x) ← p(ƒ(x)).
Draw four levels of the computation tree for the goal
← p(y).
P. Write a logic program for the predicate removeLeft, where removeLeft(A, L, X)
means that X is the list obtained from L by removing the leftmost occurrence of A
from L. For example, the prolog query
|?- removeLeft(a, [b, c, a, b, a,d], X).
returns X = [b, c, b, a, d].
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P. Let S = {a, b, c, d}. Fill in the table for a binary operation ° on S that has the
following three properties:
(1) a is a zero.
(2) b is an identity,
(3) ° is NOT associative.
°
a
b
c
d
a
b
c
d
Give an example from your table to demonstrate that ° is not associative.
P. Given the algebra 〈A; ƒ, g, c〉 where ƒ and g are unary operations and c is a
constant in A. Suppose that we are given the following two axioms.
(1) ƒ(ƒ(x)) = g(x) for all x ∈ A.
(2) g(g(x) = x for all x ∈ A.
a. Show that ƒ(g(x)) = g(ƒ(x)) for all x ∈ A.
b. Show that ƒ(ƒ(ƒ(ƒ(x)))) = x for all x ∈ A.
c. List the finite set of elements of A in the form of distinct algebraic
expressions.
P. Write down a listing of all the distinct elements in the carrier A of the algebra 〈A;
g, a, b〉, where a, b ∈ A and g3(x) = g(x) for all x ∈ A.
P. Simplify the following Boolean expression.
(b + a)(a + b)
P. Perform each of the following Boolean algebra problems.
a. Prove the equality:
b. Simplify:
c. Simplify:
d. Simplify:
!
!
! a + b + abc = a + b + c.
a + b(a + b).
(a + b) + (a b ).
!
(a + b)(b + a)(a + b).
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P. Write a recursive definition for the function “repl” where repl(A, B, L) is the list
obtained from the list L by replacing all occurrences of A in L with B. For
example,
repl(a, b, 〈a, c, a, b, d〉) = 〈b, c, b, b, d〉.
The list operations you may use are from the set {isEmpty, cons, head, tail}.
P. Complete the following sentences about relational algebra by filling in the blanks.
a. project(select(R, A, a), X) = select(project(R, X), A, a) if and only if
b. If R and S have attribute sets I and J, and I ∩ J = ∅, then Join(R, S) =
c. If R and S have attribute sets I and J with I = J, then Join(R, S) =
P. Given the algebra of natural numbers and the algebra of lists with operations as
follows, where A is an arbitrary set:
〈N; 0, isZero, succ, pred〉.
〈lists(A); 〈 〉, isEmpty, cons, head, tail〉.
Use the given operations of the algebras to recursively define each of the
following functions. You may use either equational form or if-then-else form.
a. The function count : A × Lists[A] → N to count the number of occurrences of an
element from A that is in a list. For example, count(a, 〈b, a, d, a, a〉) = 3.
b. The function ne : N × N → {true, false} defined by ne(x, y) = “x ≠ y”. In other
words, ne tests whether two natural numbers are not equal.
P. Let R and S be two relations with attributes {Name, Rank} and {Name, Specialty},
respectively that describe information about college teachers. For example, here are a few
sample entries for R and S, where the tuples are represented by rows .
R
S
Name
Rank
Name
Specialty
Jones
Smith
Brandon
Nelson
···
Lieutenant
Captain
Major
Captain
···
Jones
Smith
Brandon
Nelson
···
Pilot
Navigator
Physician
Lawyer
···
Write an expression in relational algebra to answer each of the following questions.
a. What people have the rank of Captain? The expression should evaluate to
{(Smith), (Nelson), …}.
b. What Physicians have the rank Major? The expression should evaluate to
{(Brandon), …}.
c. What are the specialties of the Captains? The expression should evaluate to
{(Navigator), (Lawyer)…}.
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P. Solve the two congruences for the smallest non-negative value:
x " 3 (mod 12)
x " 6 (mod 7)
P. Given the primes p = 3 and q = 13, let n = pq = 39 and let d = 13. Both n and d satisfy the
! requirments of the RSA algorithm. Your job is to find an encryption key e.
P. Suppose that you have the private decryption key d and for the public encryption
key (e, n) used with the RSA algorithm
a. What formula anyone can use to encrypt a number a into a number c?
b. What formula do you use decrypt c to the original message a?