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 A298758 Numbers k such that both k and 2k-1 are Poulet numbers (Fermat pseudoprimes to base 2). 4
 15709, 65281, 20770621, 104484601, 112037185, 196049701, 425967301, 2593182901, 16923897871, 32548281361, 45812984491, 52035130951, 55897227751, 82907336737, 90003640021, 92010062101, 138016057141, 204082130071, 310026150211, 620006892121, 622333751509 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 2*a(n) - 1 = A303531(n) belongs to A217465. - Max Alekseyev, Apr 24 2018 Numbers k such that both k and 2k+1 are Poulet numbers are listed in A303447. LINKS MATHEMATICA s = Import["b001567.txt", "Data"][[All, -1]]; n = Length[s]; aQ[n_] := ! PrimeQ[n] && PowerMod[2, (n - 1), n] == 1; a = {}; Do[p = 2*s[[k]] - 1; If[aQ[p], AppendTo[a, s[[k]]]], {k, 1, n}]; a (* using the b-File from A001567 *) PROG (PARI) isP(n) = (Mod(2, n)^n==2) && !isprime(n) && (n>1); isok(n) = isP(n) && isP(2*n-1); \\ Michel Marcus, Mar 09 2018 CROSSREFS Cf. A001567, A005382, A303447, A303448. Sequence in context: A134121 A177949 A204728 * A205289 A205897 A190834 Adjacent sequences:  A298755 A298756 A298757 * A298759 A298760 A298761 KEYWORD nonn AUTHOR Amiram Eldar, Jan 26 2018 STATUS approved

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Last modified September 16 07:19 EDT 2021. Contains 347469 sequences. (Running on oeis4.)