Monday, March 16, 2009

Fixed-polynomial circuit lower bounds for MA/1

In this post, we look at a recent circuit lower bound result which shows that MA/1 is not contained in the class SIZE($n^k$). Some history and motivation first: A well studied fundamental problem in lowerbouds is in showing super-polynomial circuit lowerbounds for NP. This is natural to look at, since such a result would immediately separate P and NP, because P $\in$ P/poly has poly-sized circuit families deciding languages in it.

However, though people have been looking at this problem for quite some time, even getting super-linear lower bounds on the circuit size has been evasive. While the goal has been getting super polynomial lower bounds, the current best lower bound is just $4.5n - o(n)$.

Given this state, a natural thing to do is to look at showing results for larger classes, or in other words, attack NP from above. In this line of research, Kannan showed a promising result of this nature (we have seen this in one of the homeworks): $\Sigma_2 \cap \Pi_2$ is not contained in SIZE($n^k$). Getting closer to NP, Kobler and Watanabe have shown using the halving algorithm of BCGKT (seen in class), that $ZPP^{NP}$ has a circuit lower bound of $n^k$, for any fixed $k$. This is an improvement over the previous result of Kannan, as $ZPP^{NP}$ is "closer" to NP than $\Sigma_2 \cap \Pi_2$ is (i.e. $NP \subseteq ZPP^{NP} \subseteq \Sigma_2 \cap \Pi_2$).

In a recent result, Rahul Santhanam gets even closer to NP by showing an almost polynomial circuit lower bound for MA. He shows that for any fixed $k>0$, there are languages in the class MA/1 (MA with $1$ bit of advice) which are not decided by non-uniform circuit families of size $n^k$. Why is this result important ? Intuitively, MA is like the probabilistic version of NP, and therefore such lower bounds automatically gain much value, as it is widely believed that ''randomness'' to NP does not give much more power than the basic NP class (that is, MA $=$ NP is believed to hold). This therefore nearly implies polynomial circuit lower bounds for MA, which sits just above NP in the hierarchy. We say nearly because the class for which he shows a lower bound assumes $1$ additional bit of advice (MA/1).

Now let's look at the high level picture of proving such lower bounds. Imagine we want to show a lower bound for a class C, where NP $\subseteq$ C. If it is the case that NP is not contained in P/poly, then we are done because C is only larger than NP. On the other hand, if NP $\subseteq$ P/poly, then we \emph{use} this fact to show that the polynomial hierarchy collapses to C. But there are complete problems in PH which are not decided by SIZE($n^k$) --- recall this from HW1; because of the collapse, these problems are in $C$, and this completes the proof. If we recall, we did the exact same thing for showing such lower bounds for $\Sigma_2 \cap \Pi_2$: we showed that if NP $\subseteq$ P/poly, then PH collapses to $\Sigma_2 \cap \Pi_2$. Since we know that PH has problems which are not decided by SIZE($n^k$), we have that $\Sigma_2 \cap \Pi_2$ is not contained in SIZE($n^k$).

In fact, the argument need not always be based on the two cases of whether or not NP is in P/poly. There are cases where it helps to consider a class which \emph{contains} C, and then argue depending on it's containment in P/poly. For instance, we have the result that if PSPACE $\subseteq$ P/poly, then PSPACE $=$ MA (this problem figures in HW4). Intuitively, the proof goes as follows: if PSPACE $\subseteq$ P/poly, then the prover of the IP protocol for QBF (a complete problem for PSPACE $\subseteq$ IP) can be represented by a poly-sized circuit. Merlin can therefore simply give Arthur the circuit for the correct input size and Arthur can use the circuit to simulate any query to the prover. Completeness is direct, and intuitively, soundness holds because we know no adaptive prover can cheat Arthur, so no fixed prover can! Therefore, QBF $\in$ MA.

Using the ideas defined above, we are now ready to show the main theorem that MA/1 is not in SIZE($n^k$).
We require the following lemma, whose proof is rather technical (using Shamir's proof for IP = PSPACE) and we omit.

There exists a PSPACE-complete problem $L$ and a probabilistic polynomial time turing machine $M$ such that for any input $x$,
1. $M$ asks only queries of length $|x|$.
2. If $M$ is given the language $L$ as oracle and if $x \in L$, then $M$ accepts $x$ with probability $1$.
3. If $x \notin L$, then no matter what oracle $M$ gets, it accepts with probability only at most $1/2$.

Such a complete problem is known to exist, and we shall make use of it. Just as an aside, this definition of $L$ seems similar to the notion of checkers in HW4.

If this language $L \in$ P/poly, then PSPACE $\in$ P/poly, meaning PSPACE $=$ MA, and this would give us the desired result since PSPACE is not contained in SIZE($n^k$). Therefore, we assume that $L \notin$ P/poly, and create the following \emph{padded} language $L'$. For what follows, let $s(n)$ denote the size of the smallest circuit which can decide $L$ on inputs of size $n$. Consider the following language $L'$:

$L' = \{ x1^{y} | x \in L, |x| = n, y \geq n, y$ is a power of $2, s(n) \in ((y + n)^{k+1}, (2y+n)^{k+1}] \}$.

We show that $L' \in$ MA/1 but $L' \notin$ SIZE($n^k$). We first show the latter result: In some sense, the magic is already done as the padding has sort of diagonalized the input.
Because $L \notin$ SIZE($n^k$) (we are in the case where $L \notin$ P/poly), there exists some input size $n_0$ such that $s(n_0) > (n_0+1)^k$, where $s(n)$ is the size of the smallest circuit deciding $L$ on inputs of size $n$. Define $y_0$ to be the power of $2$ such that $(n_0 + y_0)^k < s(n_0) \leq (n_0 + 2y_0)^k$. Notice that this is the same $y$ as defined in $L'$.

Now, suppose $L'$ has a circuit family of size $m^k$. We now create a "small" circuit for deciding $L$ on input size $n_0$: the circuit simply hardwires $y_0$ additional $1$'s to the input $x$ and subsequently mimics the circuit of $L'$ on inputs of size $n_0 + y$. However, the size of this circuit is at most $(n_0+y)^k$, which contradicts the definition of $s(n)$. This shows that $L' \notin$ SIZE($n^k$).

All that remains is to show that $L' \in$ MA/1. The key here is in knowing if the input size $m$ of a given $x1^y$ is \emph{valid}, in that it satisfies the circuit size condition. It would be difficult to have the MA protocol figure it out if the circuit size is large (which it probably is), but here is where we use the $1$ bit of advice. Given an input size of $m$, the advice is $1$ if and only if $m$ can be decomposed into $n$ and $y = 2^k$ such that $y \geq n$, and the smallest circuit size for deciding $L$ on inputs of size $n$ is between $(y+n)^{k+1}$ and $(2y + n)^{k+1}$. Notice that if such a valid $(n,y)$ decomposition exists, it is unique: if we try to increase $y$ to $2y$ and decrease $n$ accordingly, it would make the resulting decomposition infeasible as we have $y \geq n$.

Thus, given $x1^y$ for which we need to decide membership in $L'$, the MA protocol does the following: Merlin sends Arthur the circuit of size $s \in ((n+y)^{k+1}, (n+2y)^{k+1}]$ which solves $L$ on inputs of length $n$. Arthur checks if the advice is $1$, and rejects if it is not. If it is, then the $m$ is of a valid length, and Arthur looks at the circuit and identifies $n$ (which is the input size of the received circuit) and makes other necessary checks, like $y \geq n$, $y$ is a power of $2$, and $(y+n)^{k+1} < s \leq (2y+n)^{k+1}$. It then simulates $M$ (recall $M$, the probabilistic turing machine which decided $L$ with some oracle given), and accepts or rejects based on $M$.

It is easy to see that if $x1^y \in L'$, then Merlin could send the correct circuit, and Arthur would always accept. On the other hand, if $x1^y \notin L'$, then Merlin commits'' to an oracle by sending a circuit and by the properties of $L$ and $M$, Arthur accepts with only a small probability. This completes the proof.

1 comment:

1. Actually, the best known circuit size lower bound for the standard basis (AND, OR, NOT) and an explicit (in NP) function is 5n - o(n), due to Iwama, Lachish, Morizumi, and Raz (and their function is actually in P).