Category Combinatorics

Zarankiewicz’s Problem

Zarankiewicz’s problem plays an important role in extremal combinatorics. Let’s first learn few things about Kazimierz Zarankiewicz from Wikipedia. Unfortunately, his life had a lot of suffering. He had to live the nightmare of concentration camps during World War II and he was a revolutionary. He was not a guerrilla but continued to teach despite the Nazi […]

Recurrences and the riddle of the day

Sometimes solving exactly a recurrence can be demanding. The use of software can be useful 🙂 Something that I slept under the rag above and I want to mention it since this post by no means is supposed to be intimidating to a novice, is that in many applications (lemmas,theorems etc.) one needs the asymptotic behavior […]

Ahlswede-Zhang identity

This post is about Sperner systems and about turning the famous LYM inequality into an identity. Typically, people refer to this inequality as the LYM inequality (Lubell-Yamamoto-Meshalkin) but it is worth pointing out that the inequality follows as a corollary from the more general Bollobás inequality. While thinking of this inequality I asked myself about the cases that […]