#17. Medium-Size Intersection Is Hard To Avoid

(Back to course page.)

Link to Slides · Link to recording

Prompts for discussion:

1. Are medium-sized intersections particularly special? Are other-sized intersections also hard to avoid?

2. Here’s the statement that we used in the next miniature, whose proof is in this one, and one that I chose to skip: Let $\mathcal{F}$ be as in the theorem, i.e, $|\mathcal{F}| \leq\left(\begin{array}{l} n \\ 0 \end{array}\right)+\left(\begin{array}{l} n \\ 1 \end{array}\right)+\cdots+\left(\begin{array}{c} n \\ p-1 \end{array}\right)$. If $n=4p$, then $\frac{\left(\begin{array}{c}4 p \\2 p-1\end{array}\right)}{|\mathcal{F}|} \geq 1.1^n \text {. }$There are (apparently) many ways to prove this. Does this inequality have some visible intuition?