# Special Topics in Complexity Theory, Lecture 15

Special Topics in Complexity Theory, Fall 2017. Instructor: Emanuele Viola

### 1 Lecture 15, Scribe: Chin Ho Lee

In this lecture fragment we discuss multiparty communication complexity, especially the problem of separating deterministic and randomized communication, which we connect to a problem in combinatorics.

In number-on-forehead (NOH) communication complexity each party $i$ sees all of the input $(x_1, \dotsc , x_k)$ except its own input $x_i$. For background, it is not known how to prove negative results for $k \ge \log n$ parties. We shall focus on the problem of separating deterministic and randomizes communication. For $k = 2$, we know the optimal separation: The equality function requires $\Omega (n)$ communication for deterministic protocols, but can be solved using $O(1)$ communication if we allow the protocols to use public coins. For $k = 3$, the best known separation between deterministic and randomized protocol is $\Omega (\log n)$ vs $O(1)$ [BDPW10]. In the following we give a new proof of this result, for a simpler function: $f(x, y, z) = 1$ if and only if $x \cdot y \cdot z = 1$ for $x, y, z \in SL_2(q)$.

For context, let us state and prove the upper bound for randomized communication.

Claim 1. $f$ has randomized communication complexity $O(1)$.

Proof. In the NOH model, computing $f$ reduces to $2$-party equality with no additional communication: Alice computes $y \cdot z =: w$ privately, then Alice and Bob check if $x = w^{-1}$. $\square$

To prove a $\Omega (\log n)$ lower bound for deterministic protocols, where $n = \log |G|$, we reduce the communication problem to a combinatorial problem.

Definition 2. A corner in a group $G$ is $\{ (x,y), (xz, y), (x,zy) \} \subseteq G^2$, where $x, y$ are arbitrary group elements and $z \neq 1_G$.

For intuition, consider the case when $G$ is Abelian, where one can replace multiplication by addition and a corner becomes $\{ (x, y), (x + z, y), (x, y + z)\}$ for $z \neq 0$.

We now state the theorem that gives the lower bound.

Theorem 3. Suppose that every subset $A \subseteq G^2$ with $\mu (A) := |A|/|G^2| \ge \delta$ contains a corner. Then the deterministic communication complexity of $f(x, y, z) = 1 \iff x \cdot y \cdot z = 1_G$ is $\Omega (\log (1/\delta ))$.

It is known that when $G$ is Abelian, then $\delta \ge 1/\mathrm {polyloglog}|G|$ implies a corner. We shall prove that when $G = SL_2(q)$, then $\delta \ge 1/\mathrm {polylog}|G|$ implies a corner. This in turn implies communication $\Omega (\log \log |G|) = \Omega (\log n)$.

Proof. We saw that a number-in-hand (NIH) $c$-bit protocol can be written as a disjoint union of $2^c$ rectangles. Likewise, a number-on-forehead $c$-bit protocol $P$ can be written as a disjoint union of $2^c$ cylinder intersections $C_i := \{ (x, y, z) : f_i(y,z) g_i(x,z) h_i(x,y) = 1\}$ for some $f_i, g_i, h_i\colon G^2 \to \{0, 1\}$: \begin{aligned} P(x,y,z) = \sum _{i=1}^{2^c} f_i(y,z) g_i(x,z) h_i(x,y). \end{aligned}

The proof idea of the above fact is to consider the $2^c$ transcripts of $P$, then one can see that the inputs giving a fixed transcript are a cylinder intersection.

Let $P$ be a $c$-bit protocol. Consider the inputs $\{(x, y, (xy)^{-1}) \}$ on which $P$ accepts. Note that at least $2^{-c}$ fraction of them are accepted by some cylinder intersection $C$. Let $A := \{ (x,y) : (x, y, (xy)^{-1}) \in C \} \subseteq G^2$. Since the first two elements in the tuple determine the last, we have $\mu (A) \ge 2^{-c}$.

Now suppose $A$ contains a corner $\{ (x, y), (xz, y), (x, zy) \}$. Then \begin{aligned} (x,y) \in A &\implies (x, y, (xy)^{-1}) \in C &&\implies h(x, y) = 1 , \\ (xz,y) \in A &\implies (xz, y, (xzy)^{-1}) \in C &&\implies f(y,(xyz)^{-1}) = 1 , \\ (x,zy) \in A &\implies (x, zy, (xzy)^{-1}) \in C &&\implies g(x,(xyz)^{-1}) = 1 . \end{aligned}

This implies $(x,y,(xzy)^{-1}) \in C$, which is a contradiction because $z \neq 1$ and so $x \cdot y \cdot (xzy)^{-1} \neq 1_G$. $\square$

### References

[BDPW10]   Paul Beame, Matei David, Toniann Pitassi, and Philipp Woelfel. Separating deterministic from randomized multiparty communication complexity. Theory of Computing, 6(1):201–225, 2010.