3
$\begingroup$

For $k$ less than $n/2$, construct a bijection from the $k$-subsets of $n$ to the $(n-k)$-subsets, such that $x$ is a subset of $f(x)$.

This is a problem from a 2004 Google problem set, and answers have been posted, but could not find a good answer for this one.

2 Answers 2