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 A187787 Numbers k such that 2^(k+1) == 1 (mod k). 7
 1, 3, 15, 35, 119, 255, 455, 1295, 2555, 2703, 3815, 3855, 4355, 5543, 6479, 8007, 9215, 10439, 10619, 11951, 16211, 22895, 23435, 26319, 26795, 27839, 28679, 35207, 43055, 44099, 47519, 47879, 49679, 51119, 57239, 61919, 62567, 63167, 63935, 65535, 74447, 79055 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Prime factorizations of the first ten terms: 3, 3*5, 5*7, 7*17, 3*5*17, 5*7*13, 5*7*37, 5*7*73, 3*17*53, 5*7*109. LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 EXAMPLE 3 is in the sequence because 2^(3+1) mod 3 = 16 mod 3 = 1. MAPLE for n from 1 to 100000 do if 2&^(n+1) mod n = 1 then print(n) fi od; MATHEMATICA m = 1; Join[Select[Range[1, m], Divisible[2^(# + 1), #] &], Select[Range[m + 1, 10^5], PowerMod[2, # + 1, #] == m &]] (* Robert Price, Oct 11 2018 *) Join[{1}, Select[Range[80000], PowerMod[2, #+1, #]==1&]] (* Harvey P. Dale, Aug 19 2019 *) PROG (PARI) for (n=1, 10^7, if (Mod(2, n)^(n+1)==1, print1(n, ", "))); /* Joerg Arndt, Jan 06 2013 */ CROSSREFS Cf. A055685, A069927, A112983. Sequence in context: A015809 A293995 A015715 * A290717 A019009 A290716 Adjacent sequences: A187784 A187785 A187786 * A187788 A187789 A187790 KEYWORD nonn AUTHOR Franz Vrabec, Jan 06 2013 EXTENSIONS Term a(1)=1 prepended by Max Alekseyev, Nov 29 2014 STATUS approved

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Last modified April 1 09:11 EDT 2023. Contains 361685 sequences. (Running on oeis4.)