#17. Medium-Size Intersection Is Hard To Avoid
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Prompts for discussion:
Are medium-sized intersections particularly special? Are other-sized intersections also hard to avoid?
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?
