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A187787 Numbers n such that 2^(n+1) == 1 (mod n). 4
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

List of prime factorizations of the first ten a(n):

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

Table of n, a(n) for n=1..42.

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, 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 August 5 05:22 EDT 2020. Contains 336209 sequences. (Running on oeis4.)