Solution
= Solution
By the <Korselt criterion>, the <prime> factors are distinct. There cannot be just one <prime> factor, since a <Carmichael number> is composite. If there were exactly two, write $n=pq$ with $p<q$. Then $q-1\mid pq-1=p(q-1)+(p-1)$, so $q-1\mid p-1$, impossible because $0<p-1<q-1$. Hence \b[at least three distinct <prime> factors are necessary].