By 2x∣A, note that A=4⋅p2p2⋯pnpn. We may suppose that x=2α1p2α2⋯pnαn, where 0≤α1≤1, 0≤αi≤pi (i=2,3,…,n). Then, we have
xA=22−α1p2p2−α2⋯pnpn−αn.
Hence, the number of different divisors of xA is
(3−α1)(p2−α2+1)⋯(pn−αn+1).
We know that
(3−α1)(p2−α2+1)⋯(pn−αn+1)=x=2α1p2α2⋯pnαn.1◯
By induction on n, we shall prove that the array (α1,α2,…,αn) satisfying 1◯ is (1,1,…,1) (n≥2).
(1) If n=2, then 1◯ becomes (3−α1)(4−α2)=2α13α2, where α1∈{0,1}. If α1=0, then 3(4−α2)=3α2 which has no integer solution α2. If α1=1, then 2(4−α2)=2⋅3α2. We have α2=1. Thus, (α1,α2)=(1,1). That is, the conclusion is true for n=2.
(2) Suppose that the conclusion is true for n=k−1 (k≥3), then when n=k, 1◯ becomes
(3−α1)(p2−α2+1)⋯(pk−1−αk−1+1)(pk−αk+1)2◯=2α1p2α2⋯pk−1αk−1pkαk.
If αk≥2, considering
0<pk−αk+1<pk,
0<pi−αi+1≤pi+1<pk(1≤i≤k−1),
we see that the left-hand side of 2◯ cannot be divided by pk, but the right-hand side of 2◯ is a multiple of pk, which is a contradiction.
If αk=0, then 2◯ becomes
(3−α1)(p2−α2+1)⋯(pk−1−αk−1+1)(pk+1)3◯=2α1p2α2⋯pkαk.
Note that p2,p3,…,pk are odd primes, thus, on the one hand, pk+1 is even. So the left-hand side of 3◯ is even. On the other hand, the right side of 3◯ is odd. So α1=1. But then 3−α1=2, so the left-hand side of 3◯ is a multiple of 4, but the right-hand side of 3◯ is not, which is a contradiction.
By the above argument, we must have αk=1, and in 2◯,
pk−αk+1=pkαk=pk.
Thus,
(3−α1)(p2−α2+1)⋯(pk−1−αk−1+1)=2α1p2α2⋯pk−1αk−1.
By the induction hypotheses, α1=α2=⋯=αk−1=1.
Thus, α1=α2=⋯=αk−1=αk=1, that is, the conclusion is true for n=k.
By (1) and (2), we conclude that (α1,α2,…,αn)=(1,1,…,1), so the integer required is x=2p2⋯pn=p1p2⋯pn.