Number theory › Divisibility › Assorted divisibility puzzles

Minimum density of a special set

University · ●●●●● 200 pts

Let SS be a set of positive integers satisfying the following two conditions: (i) for each positive integer nn, at least one of n,2n,…,100nn, 2n, \ldots, 100n is in SS; (ii) if a1,a2,b1,b2a_1, a_2, b_1, b_2 are positive integers such that gcd⁡(a1a2,b1b2)=1\gcd(a_1a_2, b_1b_2)=1 and a1b1,a2b2∈Sa_1b_1, a_2b_2 \in S, then a2b1,a1b2∈Sa_2b_1, a_1b_2 \in S. Suppose that SS has natural density rr, meaning lim⁡n→∞1n∣S∩{1,…,n}∣=r\lim_{n\to\infty}\frac{1}{n}|S\cap\{1,\ldots,n\}| = r. Compute the minimum possible value of ⌊105r⌋\lfloor 10^5 r\rfloor.

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p-adic valuationmultiplicative functionsdivisor functionfloor function