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Erdős problem 442

Let Log⁡x:=max⁡{log⁡x,1}\operatorname{Log} x := \max\{\log x, 1\}, Log⁡2x=Log⁡(Log⁡x)\operatorname{Log}_2x = \operatorname{Log} (\operatorname{Log} x), and Log⁡3x=Log⁡(Log⁡(Log⁡x)).\operatorname{Log}_3x = \operatorname{Log}(\operatorname{Log}(\operatorname{Log} x)). Is it true that if A⊆NA\subseteq\mathbb{N} is such that 1Log⁡2x∑n∈A:n≤x1n→∞ \frac{1}{\operatorname{Log}_2 x} \sum_{n\in A: n\leq x} \frac{1}{n}\to\infty then (∑n∈A:n≤x1n)2∑n,m∈A:n<m≤x1lcm⁡(n,m)→∞ \left(\sum_{n\in A: n\leq x} \frac{1}{n}\right)^2 \sum_{n, m \in A: n < m \leq x} \frac{1}{\operatorname{lcm}(n, m)}\to\infty as x→∞x\to\infty?

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FormalConjectures/ErdosProblems/

442.lean

Retained formal statement1 of 2

Let Log⁡x:=max⁡{log⁡x,1}\operatorname{Log} x := \max\{\log x, 1\}, Log⁡2x=Log⁡(Log⁡x)\operatorname{Log}_2x = \operatorname{Log} (\operatorname{Log} x), and Log⁡3x=Log⁡(Log⁡(Log⁡x)).\operatorname{Log}_3x = \operatorname{Log}(\operatorname{Log}(\operatorname{Log} x)). Is it true that if A⊆NA\subseteq\mathbb{N} is such that 1Log⁡2x∑n∈A:n≤x1n→∞ \frac{1}{\operatorname{Log}_2 x} \sum_{n\in A: n\leq x} \frac{1}{n}\to\infty then (∑n∈A:n≤x1n)2∑n,m∈A:n<m≤x1lcm⁡(n,m)→∞ \left(\sum_{n\in A: n\leq x} \frac{1}{n}\right)^2 \sum_{n, m \in A: n < m \leq x} \frac{1}{\operatorname{lcm}(n, m)}\to\infty as x→∞x\to\infty?

Tao [Ta24b] has shown this is false.

[Ta24b] Tao, T., _Dense sets of natural numbers with unusually large least common multiples_. arXiv:2407.04226 (2024).

Note: the informal and formal statements follow the solution paper https://arxiv.org/pdf/2407.04226

FormalConjectures/ErdosProblems/442.leanErdos442.erdos_44210 linesExact file
False ↔  ∀ (A : Set ℕ),    Filter.Tendsto        (fun x => 1 / Erdos442.Real.maxLogOne (Erdos442.Real.maxLogOne x) * ∑ n ∈ (A ∩ Set.Icc 1 ⌊x⌋₊).toFinset, 1 / ↑n)        Filter.atTop Filter.atTop →      Filter.Tendsto        (fun x =>          1 / (∑ n ∈ (A ∩ Set.Icc 1 ⌊x⌋₊).toFinset, 1 / ↑n) ^ 2 *            ∑ nm ∈ (Erdos442.Set.bddProdUpper A x).toFinset, 1 / ↑(nm.1.lcm nm.2))        Filter.atTop Filter.atTop
SolvedStatement only, no proof