A177024 Numbers k such that 2^(k-1) mod k = number of divisors of k.
15, 21, 24, 33, 39, 40, 51, 57, 69, 87, 93, 111, 123, 129, 141, 154, 159, 177, 183, 201, 213, 219, 237, 249, 267, 291, 303, 309, 321, 327, 339, 381, 393, 411, 417, 447, 453, 471, 489, 501, 519, 537, 543, 573, 579, 591, 597, 633, 669, 681, 687, 699, 717, 723, 731, 753, 771, 789, 807, 813, 831, 843, 849, 879, 921
Offset: 1
Keywords
Links
- Amiram Eldar, Table of n, a(n) for n = 1..10000
Programs
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Mathematica
Select[Range[1000], Mod[2^(# - 1), #] == Length[Divisors[#]] &] Select[Range[1000],PowerMod[2,#-1,#]==Length[Divisors[#]]&] (* Harvey P. Dale, Nov 19 2015 *) Select[Range[1000], PowerMod[2, #-1, #] == DivisorSigma[0, #] &] (* Amiram Eldar, Jul 12 2022 *)