Chapter 18 Random Variables784
- The variables in the three terms in each proposition are all different.
Suppose that we assign true/false values to the variablesx 1 ;:::;x 9 indepen-
dently and with equal probability.
(a)What is the expected number of true propositions?Hint:LetTibe an indicator for the event that thei-th proposition is true.
(b)Use your answer to prove that foranyset of 7 propositions satisfying the
conditions 1. and 2., there is an assignment to the variables that makes all 7 of the
propositions true.
Problem 18.23.
Aliteralis a propositional variable or its negation. Ak-clauseis anORofkliterals,
with no variable occurring more than once in the clause. For example,
PORQORRORV;is a 4-clause, but
VORQORXORV;
is not, sinceVappears twice.
LetSbe a sequence ofndistinctk-clauses involvingvvariables. The variables
in differentk-clauses may overlap or be completely different, sokvnk.
A random assignment of true/false values will be made independently to each of
thevvariables, with true and false assignments equally likely. Write formulas inn,
k, andvin answer to the first two parts below.
(a)What is the probability that the lastk-clause inSis true under the random
assignment?
(b)What is the expected number of truek-clauses inS?(c)A set of propositions issatisfiableiff there is an assignment to the variables
that makes all of the propositions true. Use your answer to part (b) to prove that if
n < 2k, thenSis satisfiable.
Problem 18.24.
There arenstudents who are both taking Math for Computer Sience and Introduc-
tion to Signal Processing this term. To make it easier on themselves, the Professors
in charge of these classes have decided to randomly permute their class lists and