Erdős Problem #1000 — Sequences with Vanishing Average Reduced-Denominator Count

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For a strictly increasing sequence of positive integers A=(nk)k≥0A=(n_k)_{k\geq0}, define ϕA(k)\phi_A(k) to be the number of integers mm with 1≤m≤nk1\leq m\leq n_k such that

nkgcd⁡(m,nk)≠nj\frac{n_k}{\gcd(m,n_k)}\neq n_j

for every j<kj<k. Equivalently, m/nkm/n_k does not have denominator njn_j in lowest terms for any j<kj<k. Define

ϕavg(N)=1N∑k=0N−1ϕA(k)nk.\phi_{\mathrm{avg}}(N)=\frac1N\sum_{k=0}^{N-1}\frac{\phi_A(k)}{n_k}.

Does there exist a strictly increasing sequence of positive integers A=(nk)A=(n_k) such that

lim⁡N→∞1N∑k=0N−1ϕA(k)nk=0?\lim_{N\to\infty}\frac1N\sum_{k=0}^{N-1}\frac{\phi_A(k)}{n_k}=0?
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