The number of distinct positive divisors of an integer N=∏piei with distinct primes pi is equal to ∏(ei+1). Because 8 has only the factorizations 8, 4⋅2 and 2⋅2⋅2, we have to consider three cases:
Case A: N=p7
Case B: N=p3q
Case C: N=pqr,
where p,q,r are distinct primes. and these reduce to sub-cases.
In Case A we have dk=pk−1 and 2d2d5=d3d4+3 is equivalent to p5=3, which does not hold for any p.
In Case B we consider the following four sub-cases
1) q<p
2) p<q<p2
3) p2<q<p3
4) p3<q.
These inequalities lead to identification of d2,d3,d4,d5 in each case as follows:
1) d2=q, d3=p, d4=pq, d5=p2
2) d2=p, d3=q, d4=p2, d5=pq
3) d2=p, d3=p2, d4=q, d5=p3
4) d2=p, d3=p2, d4=p3, d5=q
These give rise to one solution from sub-case 4) with p=3 and q=41 leading to N=33⋅41=1107. The first three cases would require 3 to be divisible by p2, which is not possible.
In Case C, w.l.o.g., we can assume p<q<r. We consider two sub-cases:
1) r<pq and 2) pq<r.
1) d2=p, d3=q, d4=r, d5=pq
2p2q=qr+3, i.e. q(2p2−r)=3 and so q=3 and 2p2=r+1. As p<q we get p=2 and so r=7. However, we now have r=7>6=pq and no solutions arise.
2) d2=p, d3=q, d4=pq, d5=r
2pr=pq2+3, i.e. p(2r−q2)=3. This gives p=3. We have a solution if r=(q2+1)/2. Using q=5,7,11 gives r=13,25,61, respectively. As 13<pq=15 and 25 is not prime, the smallest value of q to yield a solution is q=11. This gives N=2013.
Larger values of q for which (q2+1)/2 is a prime (greater than 61) yield other values of N which will be bigger than 1107 and 2013. Thus these are the two smallest solutions.