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\centerline {\Large \sc Information and coding theory}
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\centerline {\underline{\Large Exercise sheet n$^\circ$ 4}:}
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\noindent {\bf 4-1.} {\em Lempel-Ziv code.}
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\noindent (a) Consider a source with alphabet $\{\mathtt{A}, \mathtt{B}, \mathtt{C}, \mathtt{\_} \}$. Encode the sequence $\mathtt{AA\_ABABBABC\_ABABC}$ with the Lempel-Ziv code. Establish a method to encode the position, and calculate the number of bits that are necessary to transmit this sequence compressed by Lempel-Ziv. What do you encounter at the last $ABC$? What would you encounter if the sequence was $\mathtt{AA\_ABABBABC\_ACABC}$?
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\noindent (b) Consider a source with alphabet $\{\mathtt{A},\mathtt{B} \}$. Encode the sequence $\mathtt{ABAAAAAAAAAAAAAABB}$ with the Lempel-Ziv code. Calculate the number of bits necessary to transmit the sequence compressed by Lempel-Ziv and compare it with a naive encoding.
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\noindent (c) Consider a source with alphabet $\{\mathtt{A},\mathtt{B}\}$. Encode the sequence $\mathtt{ABAAAAAAAAAAAAAAAAAAAA}$ with the Lempel-Ziv code. Calculate the number of bits necessary to transmit the sequence compressed by Lempel Ziv and compare it with a naive encoding.
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\noindent (d) The following is the encoded sequence that is obtained by using the Lempel-Ziv code. Reconstruct the original sequence.
\begin{center}
$0, \mathtt{A}$ \\
$1, \mathtt{B}$ \\
$2, \mathtt{C}$ \\
$0, \mathtt{\_}$ \\
$2, \mathtt{B}$ \\
$0, \mathtt{B}$ \\
$6, \mathtt{B}$ \\
$7, \mathtt{B}$ \\
$0, \mathtt{.}$
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\noindent {\bf 4-2.} Each object of an ensemble of $n$ objects can either be faulty or not. The random variable $X_i$ is either 1 or 0, respectively, if the $i$-th object is faulty or not. We assume that the variables $X_1$, $X_2, \cdots ,X_n$ are independent, with ${\rm Prob}\{X_i=1\}=p_i$ and $p_1>p_2>\cdots >p_n>1/2$.
The problem is to determine the ensemble of all faulty objects with a method that is optimal.
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\noindent (a)
How can one find the optimal sequence of yes/no-questions that determines all objects that are faulty?
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\noindent (b)
What is the last question that has to be asked? Which two ensembles can one distinguish with this question?
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\noindent {\bf 4-3.} A player $A$ choses an object from an ensemble and a player $B$ tries to identify this object with a series of yes/no-questions. We assume that player $B$ uses an optimal code (which gives the minimal expected length) with respect to the distribution of objects of player $A$. One observes that, on average, player $B$ needs to ask 35 questions to determine the object chosen by $A$. Find a lower bound on the number of objects of the ensemble.
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\noindent {\bf 4-4.} One is given $n$ coins, where one of them can be counterfeited. A counterfeited coin is either heavier or lighter then an authentic one. In order to find the counterfeited coin, all coins are weighed.
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\noindent (a) What is the probability distribution of the counterfeited coin that maximizes the Shannon entropy? Treat the existence or non-existence of a counterfeited coin as an extra element in the probability distribution.
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\noindent (b) Establish the equation that bounds from below and above the expected number of weighs, if one uses an optimal method.
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\noindent (c) Deduce the maximal number of coins $n$, given an average number of weighs $k$.
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\noindent (d) Show that this maximum can be attained.
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\noindent {\bf 4-5.} A random variable $X$ can take $m$ possible values and has an entropy $H(X)$. Suppose one is given an instantaneous code $C$ for the source $X$, which has an expected length $L(C)=H(X)/\log_2 5=H_5(X)$.
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\noindent (a) Show that each symbol of $X$ has a probability of the form $5^{-n}$, where $n$ is a whole number.
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\noindent (b) Show that $m$ has to satisfy $m=4k+1$ where $k$ is a whole number.
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\centerline {\underline{\Large Web site}:}
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http://quic.ulb.ac.be/teaching/

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