14. The smallest two n that satisfy the condition are n=2,6. Based on this analysis, we prove the following strengthened proposition: there exist infinitely many positive even numbers n1<n2<⋯ such that for any k∈N∗, we have nk∣(2nk+2), and (nk−1)∣(2nk+1).
We use mathematical induction to prove this proposition. Let n1=2, and for any k∈N∗, let nk+1=2nk+2. We claim that the sequence defined in this way satisfies the proposition.
When k=1, it is clearly true. Assume that nk∣(2nk+2) and (nk−1)∣(2nk+1). Then nk+1=2nk+2 is even, and 2nk+2=nk⋅q, where q is a positive odd number. Furthermore, let 2nk+1=(nk−1)⋅p, where p is a positive odd number. We have
2nk+1+2=2(2nk+1−1+1)=2(2(nk−1)p+1)=2(2nk−1+1)M=(2nk+2)M=nk+1⋅M
where M∈N∗ (by factorization). Thus, nk+1∣(2nk+1+2).
On the other hand, we also have
2nk+1+1=2nk⋅q+1=(2nk+1)⋅N
where N is a positive integer. Thus, (nk+1−1)∣(2nk+1+1). Therefore, the strengthened proposition holds.