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\begin{document}
\header{6.841/18.405J Advanced Complexity Theory}{March 3, 2003}
{7: $\text{BPP} \subseteq \text{PH}$, Circuit Lower Bounds}
{Prof. Madhu Sudan}{Jonathan Derryberry}
\section{$\text{BPP} \subseteq \text{PH}$}
A review of the currently known relationships among relevant complexity
classes appears in Figure~\ref{classes_fig}.
\begin{figure}[!ht]
\begin{center}
\scalebox{1.0}{\includegraphics{l7-classes_fig.eps}}
\end{center}
\caption{The currently known relationships among complexity classes.}
\label{classes_fig}
\end{figure}
The purpose of this part of the lecture is to demonstrate that
$\text{BPP} \subseteq \text{PH}$, in particular that
$\text{BPP} \subseteq \Sigma_2^{\text{P}}$.
Before proceeding with the proof, we will get an idea of what we
are trying to do. For an arbitrary language $L \in \text{BPP}$, we
are trying to show that a string $w$ is in $L$ exactly when
$\exists x_1, \ldots, x_n
\forall y_1, \ldots, y_n
P(x_1, \ldots, x_n, y_1, \ldots, y_n) = 1$ for some
polynomial time computation $P$.
In other words, the ``x player'' is capable of finding a sequence of $x$s
such that for any sequence of $y$s that the ``y player'' chooses, the polynomial
time ``audience'' will accept.
Suppose $M$ is a BPP machine for $L$.
The above characterization of membership in $\Sigma_2^{\text{P}}$ raises three
questions that the proof of $\text{BPP} \subseteq \Sigma_2^{\text{P}}$
must answer in constructing such a ``debate system'' using $M$:
\begin{enumerate}
\item What should the x player do?
\item What should the y player do?
\item What should $P$ be?
\end{enumerate}
Using $M$, on input $w$ (with $|w| = n$)
consider all possible random strings $z$ (with $|z| = m = poly(n)$)
that could be used.
Define a ``bad string'' to be a string $z$ that causes $M$ to make
the wrong decision about whether $w \in L$ (e.g. $M(w, z) = 0$ but $w \in L$).
Note that because $M$ is a BPP machine for $L$, it is the case that
if $w \in L$ then $\Pr[M(w, z) = 1] > 1 - 2^{-n}$,
and if $w \notin L$ then $\Pr[M(w, z) = 1] < 2^{-n}$.
Thus, there are very few bad random strings $z$ on a particular input $w$.
Let us consider partitioning the universe of all possible random strings
of length $m$ into two halves. One half contains only good strings and one
half contains all of the bad strings (See Figure~\ref{partition_fig}).
\begin{figure}[!ht]
\begin{center}
\scalebox{1.0}{\includegraphics{l7-partition_fig.eps}}
\end{center}
\caption{A schematic of a partition of the universe of random strings of length $m$.}
\label{partition_fig}
\end{figure}
If the random string $z$ falls in the good partition then $M$ gives the correct
answer, while if $z$ falls in the bad partition there is a small probability of
error. Thus, if an efficiently computable bijection $\pi: \{0, 1\}^m \mapsto \{0, 1\}^m$
is created between the two halves, then the results
of both $M(w, z)$ and $M(w, \pi(z))$ can be examined with assurance that one of them
is correct and one may be incorrect with low probability.
Thus, if $w \in L$ then $M(w, z) = 1$ or $M(w, \pi(z)) = 1$.
Note that the x player can announce this bijection $\pi$ in its round,
and the y player can check whether $\pi$ is, in fact, a bijection by
testing all possible inputs to the bijection and exhibiting a pair that map to
the same value to the audience if x tried to cheat by announcing a function
that was not a bijection.
One problem with this is that it is not clear that such an efficiently
computable bijection exists.
One candidate bijection is the family of bijections
\begin{equation*}
\pi_r : z \mapsto z \oplus r,
\end{equation*}
where $r \in \{0, 1\}^m$.
It is clear that $\pi_r$ is a bijection and is
efficiently computable; however it is not clear that it creates a good
partition. Nevertheless, on average we expect that using $\pi_r$ for a
particular random $r$ will cause $\Pr[M(w, z) \vee M(w, \pi_r(z)) = 0]$
to be much smaller than $\Pr[M(w, z) = 0]$ given that $w \in L$ because
the probability that both $z$ and $\pi_r(z)$ are bad is less than $2^{-n} \cdot 2^{-n}$,
where $n$ is the length of $w$.
Clearly, this represents an improvement so let us strengthen the improvement by
choosing $r_1, \ldots, r_l$ at random and fixing $z$ to some random value.
Continuing the argument from above, we have
\begin{equation*}
\Pr[M(w, \pi_{r_1}(z)) \vee \cdots \vee M(w, \pi_{r_l}(z)) = 0]
< (2^{-n})^l,
\end{equation*}
which is less than $2^{-m}$ for some $l \leq m$.
Note that this probability is so small that it is less than the probability
of choosing a particular $m$ bit string at random.
Also, note that if $w \notin L$ then the probability of making an error has
increased, but only linearly in $l$ because
\begin{equation*}
\Pr[M(w, \pi_{r_1}(z)) \vee \cdots \vee M(w, \pi_{r_l}(z)) = 1]
< l \cdot 2^{-n},
\end{equation*}
Now, to answer the three questions from above,
consider what happens if
\begin{enumerate}
\item The x player uses the strings $r_1, \ldots, r_l$.
\item The y player uses all values of $z$.
\item The audience uses $M(w, \pi_{r_1}(z)) \vee \cdots \vee M(w, \pi_{r_l}(z))$.
\end{enumerate}
That is, we consider the formula
\begin{equation}
\label{sigma2_eq}
\exists r_1, \ldots, r_l \forall z M(w, \pi_{r_1}(z)) \vee \cdots \vee M(w, \pi_{r_1}(z)).
\end{equation}
To see that Equation~\ref{sigma2_eq} proves that $L \in \Sigma_2^{\text{P}}$,
note that $M(w, \pi_{r_i}(z))$ is a polynomial time computation and it is repeated
$l$ times, which is also polynomial in $n$. Also, note that it is correct because:
\begin{itemize}
\item If $w \in L$, then the probability that for random $r_1, \ldots, r_l$
$M(w, \pi_{r_1}(z)) \vee \cdots \vee M(w, \pi_{r_l}(z))$ gives the wrong answer
is so small that less than one potential $z$ (member of $\{0, 1\}^m$)
causes an error on average.
Thus, there must be some choice of $r_1, \ldots, r_l$ such that no error occurs
for any $z$.
That is,
\begin{equation*}
w \in L \implies \exists r_1, \ldots, r_l \forall z
M(w, \pi_{r_1}(z)) \vee \cdots \vee M(w, \pi_{r_1}(z)) = 1.
\end{equation*}
\item If $w \notin L$, then for random $r_1, \ldots, r_l$
$\Pr[M(w, \pi_{r_1}(z)) \vee \cdots \vee M(w, \pi_{r_1}(z)) = 1] < l \cdot 2^{-n}$
as shown above. Thus, for {\em any} setting of $r_1, \ldots, r_l$
there is some $z$ (in fact, there are many) such that
$M(w, \pi_{r_1}(z)) \vee \cdots \vee M(w, \pi_{r_1}(z)) = 0$.
In other words,
\begin{equation*}
w \notin L \implies \forall r_1, \ldots, r_l \exists z
M(w, \pi_{r_1}(z)) \vee \cdots \vee M(w, \pi_{r_1}(z)) = 0.
\end{equation*}
This is just another way of stating
\begin{equation*}
w \notin L \implies \neg \exists r_1, \ldots, r_l \forall z
M(w, \pi_{r_1}(z)) \vee \cdots \vee M(w, \pi_{r_1}(z)) = 1.
\end{equation*}
\end{itemize}
\section{Circuit Lower Bounds}
To motivate the discussion of circuit lower bounds, consider the question
of whether it is possible to get rid of the randomness in randomized algorithms
and still have them recognize the same languages.
Randomness can be simulated by constructing or computing strings that
look random to polynomial time machines or, say, time $n^2$ machines.
An important component of this progress involves proving {\em circuit lower bounds}.
Recall that a circuit is a directed acyclic graph with:
\begin{itemize}
\item $n$ {\em input} nodes, which have in-degree 0, arbitrary out-degree,
and are labeled $x_1, \ldots, x_n$
\item 1 {\em output} node, which has arbitrary in-degree and out-degree 0
\item many {\em internal} nodes, which have 3 types of functions:
\begin{enumerate}
\item NOT, which has in-degree 1 and arbitrary out-degree
\item OR, which has arbitrary in-degree and arbitrary out-degree
\item AND, which has arbitrary in-degree and arbitrary out-degree
\end{enumerate}
\end{itemize}
For our purposes, there are two important measures of circuits:
\begin{enumerate}
\item The SIZE of a circuit is the number of edges in its graph.
\item The DEPTH of a circuit is the length of the longest path in its graph.
\end{enumerate}
One universal bound on circuit size is that for all functions
$f:\{0, 1\}^n \mapsto \{0, 1\}$, there is a circuit of size $n2^n$ that
computes $f$. To see that this is true, consider the circuit that has the
entire table of values for $f$ encoded as a sum of products (DNF).
\begin{proposition}
There exists a function $f$ that cannot be computed by a circuit of size
$2^n/n$.
\end{proposition}
The typical function for which lower bounds are proven is
$f(h, x) = h(x)$, which splits the input into a function $h$ and
an input $x$ for $h$.
In this way, diagonalization can be used to prove that there is
some $\{f_n\}$, computable in PSPACE that requires a size $n \log n$ circuit.
If C is a circuit of size $s$ and depth $d$ that computes
$x_1 \oplus \cdots \oplus x_n$ (i.e. parity) then $s \geq 2^{n^{\Omega(1/d)}}$.
\end{document}