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Unformatted text preview: EE595A Introduction to Information Theory University of Washington Winter 2004 Dept. of Electrical Engineering Handout 8: Problem Set 4: Solutions Prof: Jeff A. Bilmes <bilmes@ee.washington.edu> Lecture 10, March 10, 2004 Book Problems Do problems 7.4(abcd),7.6,7.9,8.8,8.10,8.12,9.1,10.2 For problem 7.4d, compute the probability of the sequence 1 2 . . . n which is then followed by any arbitrary se quence, where = X p : U ( p ) halts 2 l ( p ) , and = . 1 2 . . . . Also, do not do 7.4e. (note: a good break in this problem set is to be done with all chapter 7 problems, problem 1 and 2 below, and 1/2 of the chapter 8 problems by Friday). Problem 7.4 Monkeys on a computer. Suppose a random program is typed into a computer. Give a rough estimate of the probability that the computer prints the following sequence: 1. n followed by any arbitrary sequence. 2. 1 2 . . . n followed by any arbitrary sequence, where i is the ith bit in the expansion of . 3. n 1 followed by any arbitrary sequence. 4. (Optional) 1 2 . . . n followed by any arbitrary sequence, where = U ( p ) halts 2 l ( p ) , = . 1 2 . . . . Solution 7.4 The probability that a computer with a random input will print will print out the string x followed by any arbitrary sequence is the sum of the probabilities over all sequences starting with the string x . p U ( x . . . ) = X y { , 1 } * { , 1 } p U ( xy ) , where p U ( x ) = X p : U ( p )= x 2 ( p ) . (8.1) This sum is lower bounded by the largest term, which corresponds to the simplest concatenated sequence. 1. The simplest program to print a sequence that starts with n 0s is Print 0s forever. This program has constant length c and hence the probability of strings starting with n zeroes is p U (0 n . . . ) 2 c . (8.2) 2. Just as in part (a), there is a short program to print the bits of forever. Hence p U ( 1 2 . . . n . . . ) 2 c . (8.3) 3. A program to print out n 0s followed by a 1 must in general specify n . Since most integers n have a complexity log * n , and given n , the program to print out n 1 is simple, we have p U (0 n 1 . . . ) 2 log * n c , (8.4) 81 82 4. We know that n bits of are essentially incompressible, i.e., their complexity n c . Hence, the shortest program to print out n bits of followed by anything must have a length at least n c , and hence p U ( 1 2 . . . n . . . ) 2 ( n c ) . (8.5) Problem 7.6 Do computers reduce entropy? Let X = U ( P ) , where P is a Bernoulli(1/2) sequence. Here the binary sequence X is either undefined or is in { , 1 } * . Let H ( X ) be the Shannon entropy of X . Argue that H ( X ) = ....
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This note was uploaded on 10/12/2009 for the course EE 596A taught by Professor Jeffa.bilmes during the Winter '04 term at Washington University in St. Louis.
 Winter '04
 JeffA.Bilmes
 Electrical Engineering

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