Monday, March 05, 2007

Homework correction + the Ajtai-Linial Function

First, Aaron correctly pointed out a mistake on the problem set. In 4a, you should also assume the algorithm has query access to $f$. The corrected version of the homework is on the course web page.

Now for the real topic of the post. Recall that KKL proved that if $f : \{-1,1\}^n \to \{-1,1\}$ is any roughly balanced function, there is coalition $J \subseteq [n]$ of size $O(n / \log n)$ with influence .999 on $f$.

A construction of Ajtai and Linial (The influence of large coalitions, Combinatorica 13, 1993; annoyingly, not available online) shows this is close to optimal.

The [AL] construction is randomized. Recall the Tribes function, which breaks up the input into disjoint blocks ("tribes") of size about $\log n - C \log \log n$ and takes the OR across blocks of the AND of the blocks. Here $C$ is a carefully chosen constant (around $1$) picked so that the overall function is roughly balanced.

Let's say a random nonmonotone Tribes function of width $w$ is given by randomly partitioning $[n]$ into blocks of size $w$, randomly choosing whether to negate each variable (50-50), and then again ANDing within blocks and ORing across blocks.

Choose $w$ so that these functions will be True with probability around $1 - (\ln 2)/n$. (This involves taking $C \sim 2$.)

Finally, the [AL] construction is this: Choose $n$ random nonmonotone Tribes functions with this width, and AND them together.

--------

Here is a sketch of the properties of this (randomly chosen) function:

1. The probability it is True is very close to $1/2$, with very high probability.

2. It's hard for small coalitions to force the function towards True. Because to do this, they have to force many of the random Tribes functions towards True, and for each such function ($\sim n$ of them), they have to force one of its Tribes (about $\log n$ variables) towards True. By the random construction, the different Tribes don't overlap much, so the coalition has to be nearly linear in size.

3. It's hard for small coalitions to force the function towards False. To do this, they have to force at least one of the Tribes functions towards False, and this requires fixing at least one variable in each tribe within the function ($\sim n/\log n$ variables).

--------

The actual theorem is that with high probability over the choice of $f$, $f$ is near-balanced and every coalition $J$ of size $o(n/\log^2 n)$ has influence $o(1)$ on $f$.

Oddly enough, the hardest part of the proof is showing that the functions are near-balanced with very high probability.

--------

Some open problems:

1. Close the gap between $n/\log^2 n$ and $n/\log n$. [AL] speculated that the latter is probably the truth, but I'm not so sure...

2. Give an explicit function where all $o(n/\log^2 n)$-size coalitions have $o(1)$ influence. In fact, I believe it's open even to do this for $c \sqrt{n}$-size coalitions for large enough $c$.