The EKG prime-neighbor conjecture

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Let (a(n))n≥1(a(n))_{n\geq 1} be the EKG sequence defined by a(1)=1a(1)=1, a(2)=2a(2)=2, and, for n≥3n\geq 3, letting a(n)a(n) be the smallest natural number not already in the sequence such that gcd⁡(a(n−1),a(n))>1\operatorname{gcd}(a(n-1),a(n))>1. The EKG prime-neighbor conjecture. Whenever a prime pp occurs in the sequence, it is immediately preceded by 2p2p and hence followed by 3p3p. This is suggested by numerical computation through the first 10710^7 terms, but the claim is not proved.

References

Primary source

J. C. Lagarias, E. M. Rains and N. J. A. Sloane, “The EKG Sequence”, arXiv:math/0204011 (2002).

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