Solution:
Suppose n=pqr, where p<q are primes and r>1. Then p≥2, q≥3 and r≥2, not necessarily a prime. Thus we have
n−2≥n−p=pqr−p≥5p>pn−2≥n−q=q(pr−1)≥3q>qn−2≥n−pr=pr(q−1)≥2pr>prn−2≥n−qr=qr(p−1)≥qr
Observe that p,q,pr,qr are all distinct. Hence their product divides (n−2)!. Thus n2=p2q2r2 divides (n−2)! in this case. We conclude that either n=pq where p,q are distinct primes or n=pk for some prime p.
Case 1. Suppose n=pq for some primes p,q, where 2<p<q. Then p≥3 and q≥5. In this case
n−2>n−p=p(q−1)≥4pn−2>n−q=q(p−1)≥2q
Thus p,q,2p,2q are all distinct numbers in the set {1,2,3,…,n−2}. We see that n2=p2q2 divides (n−2)!. We conclude that n=2q for some prime q≥3. Note that n−2=2q−2<2q in this case so that n2 does not divide (n−2)!.
Case 2. Suppose n=pk for some prime p. We observe that p,2p,3p,…,(pk−1−1)p all lie in the set {1,2,3,…,n−2}. If pk−1−1≥2k, then there are at least 2k multiples of p in the set {1,2,3,…,n−2}. Hence n2=p2k divides (n−2)!. Thus pk−1−1<2k.
If k≥5, then pk−1−1≥2k−1−1≥2k, which may be proved by an easy induction. Hence k≤4. If k=1, we get n=p, a prime. If k=2, then p−1<4 so that p=2 or 3; we get n=22=4 or n=32=9. For k=3, we have p2−1<6 giving p=2; n=23=8 in this case. Finally, k=4 gives p3−1<8. Again p=2 and n=24=16. However n2=28 divides 14! and hence is not a solution.
Thus n=p,2p for some prime p or n=8,9. It is easy to verify that these satisfy the conditions of the problem.