Solution
= Solution
A <derangement> of a finite <set> $A$ is a <permutation> $\sigma:A\to A$ with \b[no fixed point]: $\sigma(a)\ne a$ for every $a\in A$. Thus every element is moved, although the <permutation> remains a <bijection>.
= Solution
A <derangement> of a finite <set> $A$ is a <permutation> $\sigma:A\to A$ with \b[no fixed point]: $\sigma(a)\ne a$ for every $a\in A$. Thus every element is moved, although the <permutation> remains a <bijection>.