association scheme

A Boolean Function with Small Degree and Many Variables

Recently, while working on a research project, I got on a tangent. From this tangent, I got on another tangent and that is what I want to write about today: a very nice Boolean function. This example got rediscovered several times for different reasons and, as I try to emphasize from time, I believe that things that are getting rediscovered many times must be of particular importance.

So let us define our Boolean function. I will give three very similar definitions throughout this post, but I will start with only one. Put {\ell(d) := 3 \cdot 2^{d-1} - 2}. Our Boolean function {F_d: \{ 0, 1\}^{\ell(d)} \rightarrow \{ 0, 1\}} is defined as follows: Put {F_1(x) = x} (as {\ell(1)=1}). For {d > 1}, write {x \in \{ 0, 1\}^{\ell(d)}} as {x=(s,t,y,z)} with {s,t \in \{ 0,1 \}} and {y, z \in \{ 0, 1\}^{\ell(d-1)}}. Then use this rule:

  • If {s=t=1}, then {F_d(1,1,y,z) = F_{d-1}(y)}.
  • If {s \neq t=0}, then {F_d(1,0,y,z) = F_{d-1}(z)}.
  • If {s= t = 0}, then {F_d(0,0,y,z) = 1-F_{d-1}(y)}.
  • If {s \neq t = 1}, then {F_d(0,1,y,z) = 1-F_{d-1}(z)}.

In the following, we list some properties of this function. Many of the concepts here are also discussed in an earlier post of mine.


A Very Short History of Pseudorandom Cliquefree Graphs

I started writing this blog post some months ago. Occasion was that my paper “A construction for clique-free pseudorandom graphs” (in joint work Anurag Bishnoi and Valentina Pepe) was accepted by Combinatorica with minor revisions. More precisely, one of the referees was unfavorable of publication because he got the impressions that we are simply restating a result by Bannai, Hao and Song. I think that the referee had a point, but for slightly wrong reasons. This triggered me to do two things. First of all, it made me include more history of the construction in our actual paper. Then I wanted to write a blog post about the history of the construction. Sadly, I wanted to include too much history in my first attempt to write this post, so it was very much out of scope. Here now a more concise version of my original plan.


The Independence Number of the Orthogonality Graph — Or: The Usefulness of Literature Study

Let {X} be the orthogonality graph, that is the graph with {\{ -1, 1 \}^n} as vertices with two vertices adjacent if they are orthogonal. So {x, y \in \{ -1, 1 \}^n} are adjacent if {x \cdot y = x_1y_1 + x_2y_2 + \ldots + x_ny_n = 0}. There are many publications which investigate this problem. The aim of this post is two fold:

  1. To summarize the state of the art.
  2. To demonstrate how careful literature study is helpful to obtain results.


Boolean Degree 1 Functions on Association Schemes

This is the announced post on my recent paper with Yuval Filmus on Boolean degree {1} functions on association schemes. The post will focus on what motivates the problem from various points of view.

Before I start, a small remark. I am using latex2wp for the first time in this post. Thanks to Luca Trevisan for providing this nice script.

During this post I avoid using many definition, while I cite things that use all kind of terminology. For this I wrote this blog post as a reference. We will go through the main steps in the paper and focus more on the wider context of the research, so the parts for which there is less space in papers and not enough time in talks.