Asymptotic for the Pell-family consecutive powerful-number progressions

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Let A\mathcal{A} be the set of positive integers NN for which some d∈Nd\in\mathbb{N} makes N,N+d,N+2dN,N+d,N+2d a three-term arithmetic progression of consecutive powerful numbers. Let A2\mathcal{A}_2 consist of those NN for which the corresponding progression contains exactly two squares. For the Pell-family progressions, write

N=(x−2)2,N+d=(x−1)2,N+2d=73y2=x2−2.N=(x-2)^2,\qquad N+d=(x-1)^2,\qquad N+2d=7^3y^2=x^2-2.

Here A=130576328A=130576328. Pell-family asymptotic conjecture. The number of such progressions with all terms in [1,n][1,n] is

(C7+o(1))log⁡n,\bigl(C_7+o(1)\bigr)\log n,

where

C7=12∏m∉{1,7}\m squarefree⁡(1−2m3/2)(log⁡(A+A2−1))−1≈0.0014.C_7=\frac{1}{2}\prod_{\substack{m\notin\{1,7\}\m\ \operatorname{squarefree}}}\left(1-\frac{2}{m^{3/2}}\right)\Bigl(\log\bigl(A+\sqrt{A^2-1}\bigr)\Bigr)^{-1}\approx0.0014.

This is a heuristic conjecture based on the recurrence and local-density considerations; no proof of the stated asymptotic is given.

References

Primary source

Wouter van Doorn, “Three-term arithmetic progressions of consecutive powerful numbers”, arXiv:2605.06697 (2026).

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