For any positive integer d, we aim to prove that there are infinitely many positive integers n such that d(n!)−1 is a composite number.
### Case 1: d=1
Assume for the sake of contradiction that for all sufficiently large n∈N, n!−1 is prime. Define pn=n!−1 to be this prime for all n≥N for some N∈N. Notice that pn>n. Consider n=pn−kn for some kn∈N.
In Fpn, by Wilson's Theorem, we have:
1=(pn−kn)!=(pn−1)(pn−2)⋯(pn−kn+1)(pn−1)!=(−1)kn−1⋅(kn−1)!−1=(−1)kn⋅(kn−1)!1
Thus,
(kn−1)!≡(−1)kn(modpn)
If we pick n≡1(mod2), then kn≡pn−n≡0(mod2) as pn is odd, and therefore pn∣(kn−1)!−1.
Since kn>C for all sufficiently large n∈N for any C∈N, and considering the pigeonhole principle, we conclude that pn=(kn−1)!−1. Hence, pn=n!−1 implies pn−n−1=kn−1=n, which means pn=2n+1. This implies 2n+1=n!−1 for infinitely many n∈N, which is impossible for large n. Therefore, n!−1 cannot be prime for all sufficiently large n.
### General Case: d≥2
Assume for the sake of contradiction that there exists some N∈N such that for all n∈N,n≥N, d⋅n!−1 is prime. Define the function f:N∖[d]→N as:
f(m)=(d−1)!⋅(d+1)(d+2)⋯(m−1)⋅m+(−1)m
Let pm be the smallest prime divisor of f(m). For m≥2d, pm>m because gcd(m!,f(m)−1)∣d but d∣f(m) implies gcd(f(m)−1,d)=1. Thus, pm∣f(m)+(−1)m, meaning:
0≡d(f(m)+(−1)m)≡m!+d(−1)m(modpm)
Using Wilson's Theorem in Fpm:
(pm−m−1)!=(pm−1)(pm−2)⋯(pm−m)(pm−1)!=(−1)m⋅m!−1=(−1)m(−(−1)m)⋅d−1=d1
Thus,
pm∣d(pm−m−1)!−1
For sufficiently large m, pm=d⋅(pm−m−1)!−1. By the pigeonhole principle, km=pm−m can appear finitely many times. Therefore, km≥N+1 implies d⋅(pm−m−1)!−1 is prime, leading to a contradiction.
Hence, for any positive integer d, there are infinitely many positive integers n such that d(n!)−1 is a composite number.
The answer is: \boxed{\text{There are infinitely many positive integers } n \text{ such that } d(n!) - 1 \text{ is a composite number.}}