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A358717 A sequence of sorted primes 2 = p_1 < p_2 < ... < p_m such that (p_i + 1)/2 divides the product p_1*p_2*...*p_(i-1) of the earlier primes and each prime factor of (p_i-1)/2 is a prime factor of the product. 2
2, 3, 5, 11, 19, 37, 73, 109, 1459, 2179, 2917, 4357, 8713 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
The sequence was used, together with A358718 and A358719, by Ferrari and Sillari (Preprint-2022) to prove that there are at least three solutions n to phi(n+k) = 2* phi(n) for all even k <= 4*10^58.
I have checked up to 10^8 and found no more terms.
Prime a(14) does not exist, which can be established by going over the divisors d of the product a(1)*...*a(13) and testing 2*d-1 as a candidate for a(14). - Max Alekseyev, Feb 19 2024
LINKS
M. Ferrari and L. Sillari, On the minimal number of solutions of the equation phi(n+k) = M*phi(n), M=1,2, arXiv:2110.05401 [math.NT], 2021.
MATHEMATICA
s = {2}; step[s_] := Module[{p = NextPrime[s[[-1]]], r = Times @@ s}, While[! Divisible[r, (p + 1)/2] || ! Divisible[r, Times @@ FactorInteger[(p - 1)/2][[;; , 1]]], p = NextPrime[p]]; Join[s, {p}]]; Nest[step, s, 12] (* Amiram Eldar, Nov 30 2022 *)
CROSSREFS
Similar to A001259.
See also A358718 and A358719.
Sequence in context: A037082 A084573 A155954 * A337347 A087581 A360320
KEYWORD
nonn,full,fini
AUTHOR
Lorenzo Sillari, Nov 28 2022
EXTENSIONS
Keywords 'full' and 'fini' added by Max Alekseyev, Feb 19 2024
STATUS
approved

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Last modified March 28 05:01 EDT 2024. Contains 371235 sequences. (Running on oeis4.)