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%I #14 Dec 11 2021 14:35:22
%S 1,3,9,27,81,171,243,513,729,1539,2187,3249,4617,6561,9747,13203,
%T 13851,19683,29241,39609,41553,59049,61731,87723,118827,124659,177147,
%U 185193,250857,263169,356481,373977,531441,555579,752571,789507,1063611,1069443,1121931,1172889,1594323,1666737
%N Terms k of A006521 such that 2*k is a term of A124240.
%C Novák numbers n that are 2n Novák-Carmichael. See Kalmynin link.
%H Alexander Kalmynin, <a href="https://arxiv.org/abs/1611.00417">On Novák numbers</a>, arXiv:1611.00417 [math.NT], 2016. See Theorem 7 p. 11.
%t Reap[Do[If[PowerMod[2, n, n]+1 == n && Divisible[2n, CarmichaelLambda[2n]], Print[n]; Sow[n]], {n, 2 10^6}]][[2, 1]] (* _Jean-François Alcover_, Sep 25 2018 *)
%o (PARI) isnov(n) = Mod(2, n)^n==-1; \\ A006521
%o isnovcar(n) = n%lcm(znstar(n)[2])==0; \\ A124240
%o isok(n) = isnov(n) && isnovcar(2*n);
%o (Python)
%o from itertools import count, islice
%o from sympy.ntheory.factor_ import reduced_totient
%o def A289257gen(): return filter(lambda n:2*n % reduced_totient(2*n) == 0 and pow(2,n,n)==n-1, count(1))
%o A289257_list = list(islice(A289257gen(),20)) # _Chai Wah Wu_, Dec 11 2021
%Y Cf. A006521, A124240, A289258.
%K nonn
%O 1,2
%A _Michel Marcus_, Jun 29 2017
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