## Monday, April 21, 2008

Homework 5 is graded and will be handed back tomorrow. The average was 79%.

1. Most people got this right (one way or another).

2. For part b), the two equally simplest counterexamples are {$x_1$, $x_1$, $1$}, or {$x_1$, $x_1$, $x_1$}. Aaron, Kanat, and Mike all gave the same, far-from-simplest counterexample. What a coincidence...

3. Many people failed to prove a characterization (if and only if statement) in c), and just gave an implication. Many people also had complicated solutions for the support-3 problem. Ali and Amitabh gave roughly the same solution, which is probably the slickest. Suppose f = a $\chi_S$ + $b \chi_T$ + $c \chi_U$, where the coefficients $a, b, c$ are nonzero and $S$, $T$, $U$ are distinct. By Parseval,

$a^2 + b^2 + c^2 = 1$ (*).

Substituting in $(1, ..., 1)$, we get $a + b + c \in {-1,1}$. Squaring this and subtracting (*) yields

$ab + bc + ac = 0$ (**).

Since $S$, $T$, $U$ are distinct, there must be an index $i$ which is in exactly 1 or 2 of them. Substituting in the string which is 1 everywhere except that it's $-1$ on the $i$th coordinate, we conclude $\sigma_1 a + \sigma_2 b + \sigma_3 c \in {-1,1}$, where exactly 1 or 2 of the signs $\sigma_i$ are $-1$. Squaring this and subtracting (*) yields

$\tau_1 ab + \tau_2 bc + \tau_3 ac = 0$ (***),

where exactly two of the signs $\tau_i$ are $-1$. Now add (**) and (***) and conclude that one of $ab$, $bc$, or $ac$ must equal 0, a contradiction.

4. Most people got (a) and (b). For (c), several people estimated the degree-1 Fourier coefficients of Majority and concluded that it's, say, $(1/n, 1/2)$-quasirandom. Only Eric truly ventured to high degree, pointing out that $Inf_i(Maj)$ can be seen to be $\Theta(1/\sqrt{n})$, and therefore by 5a, Maj is $(\Theta(1/\sqrt{n}),0)$-quasirandom. In fact, one can show that the largest Fourier coefficients for Maj are the ones at level $1$, and hence Maj is $(1/n, 0)$-quasirandom.

5. Most people more or less got this.

6. Most people mostly got this; the key in b) is picking the right diameter.

7. Most people got this too, although many failed to do both parts of f).