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\begin{center} \Large
{\scshape 6.897 Advanced Data Structures (Spring'05)} \\[1ex]
{Prof.~Erik Demaine \quad\quad TA: Mihai P\v{a}tra\c{s}cu} \\[2ex]
\framebox{Problem 6} \quad {\em Due: Monday, Mar.~14}
\end{center}
In this problem, we are considering a symmetric communication game
involving Alice and Bob, in which players alternate sending $m$-bit
messages ($m$ is the same for both players; using the notation from
class, $m=a=b$). Let $t$ be the total number of messages sent. We
will be interested in optimizing $m$, for given $t$ and input size
$n$.
Alice and Bob each receive an $n$-bit number (call these $x$ and $y$),
and they must compute the greater-than function: $GT(x,y) = 1$ if
$x>y$, and $0$ otherwise. At the end of the protocol, one player (it
doesn't matter who) must announce $GT(x,y)$. This is not considered a
round; it's just the way the result of the computation is made public.
We are interested in ``public-coin'' protocols which compute
$GT(\cdot, \cdot)$ with error probability at most $\frac{1}{3}$. At
the beginning, before they see their inputs, Alice and Bob toss some
random coins, and the outcomes are known to both players. Then, they
receive $x$ and $y$ respectively, and they execute the communication
protocol. It must be true that for any fixed $x$ and $y$, the
probability over the coin tosses that $GT(x,y)$ is announced correctly
is at least $\frac{2}{3}$.
\paragraph{Upper Bound.}
Prove that one can solve the problem with $m = O(n^{1/t} \lg n)$.
{\em Hints:} Think of the van Emde Boas recursion (but note that you
need to divide integers into more than 2 chunks). Remember that if you
pick a random function from a universal family mapping some arbitrary
universe to $\lg n$ bits, the probability that two different inputs
look identical through the hash function is $\frac{1}{n}$. Alice and
Bob can pick a random hash function ahead of time (this is what
``public coins'' really means). The player who announces $GT(x,y)$
depends on the parity of $t$.
\paragraph{Lower Bound.}
Prove that the problem does not have a solution unless $m =
\Omega(n^{1/t} / t^2)$.
\medskip
You will probably want to use the round elimination lemma. For
convenience, we are restating it here. For any communication game
deciding a function $f$, we define the $k$-fold of $f$, denoted
$f^{(k)}$, as follows. Alice receives $(x_1, \dots, x_k)$ and Bob
receives $(y, i, x_1, \dots, x_{i-1})$. The players want to compute
$f(x_i, y)$.
\begin{lemma}[round elimination lemma]
Assume $f^{(k)}$ has a protocol with error probability $\delta$, in
which Alice speaks first, players send $m$ bits per message, and there
are $t$ messages in total. Then, $f$ has a protocol with error
probability $\delta + O(\sqrt{m/k})$, in which Bob speaks first,
players send $m$ bits per message, and there are $t-1$ messages in
total.
\end{lemma}
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