Consider the integer N=(11⋅31⋅41⋅61⋯pn)5−1. Then N is relatively prime to 11,41,⋯pn. Let a=11⋅31⋅41⋅61⋯pn, then
N=a5−1=(a−1)(a4+a3+a2+a+1).
Clearly, 2∤(a4+a3+a2+a+1), but 5∤(a4+a3+a2+a+1). Let p=5 be a prime factor of a4+a3+a2+a+1. Therefore, p∤(a−1). The reason is as follows:
If p∤(a−1), then a=kp+1, for some integer k, which gives
a2=(kp+1)2,a3=(kp+1)3,a4=(kp+1)4,
and
a4+a3+a2+a+1=(kp+1)4+(kp+1)3+(kp+1)2+(kp+1)+1≡5(modp).
In fact, (p−1)≡4(mod5), that is, p−1=5k+4.
By Fermat's theorem, p∤(ap−1−1). But at this point
ap−1−1=a5k+4−1=a4(a5k−1)+(a4−1),
and since (a5−1)∤(a5k−1)=(a5)k−1k, this means p∤(a5k−1). That is, p∤(a4−1). However
a5−1=a(a4−1)+(a−1)
Therefore, if p∤(a5−1) and p∤(a4−1), then p∤(a−1). The above cannot happen. By a similar approach, we obtain that the remainder of (p−1) divided by 5 cannot be 1,2,3.
Hence 5∣(p−1) and p−1 is even. We get 10∣(p−1), that is, p=10k+1, so p is one of the terms of the given arithmetic sequence. Therefore, the prime factors of a4+a3+a2+a+1 are 5 and primes of the form 10k+1.
However a4+a3+a2+a+1>5, and 52∤(a4+a3+a2+a+1). In fact, the integer a has last digit 1, that is, a=5k+1. By the binomial theorem, we get
a4+a3+a2+a+1=(5k+1)4+(5k+1)3+(5k+1)2+(5k+1)+1=5⋅[5(25k4+25k3+10k2+2k)+1].
Therefore N=a5−1 has at least one prime factor of the form 10k+1. But by the above, N is relatively prime to all numbers of the form 10k+1. This is a contradiction.