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A177024
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Numbers k such that 2^(k-1) mod k = number of divisors of k.
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1
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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
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OFFSET
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1,1
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LINKS
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Amiram Eldar, Table of n, a(n) for n = 1..10000
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FORMULA
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A062173(a(n)) = A000005(a(n)).
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MATHEMATICA
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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 *)
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CROSSREFS
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Cf. A000005, A062173, A176175.
Sequence in context: A343821 A294171 A306102 * A325037 A154545 A156063
Adjacent sequences: A177021 A177022 A177023 * A177025 A177026 A177027
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KEYWORD
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nonn
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AUTHOR
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Juri-Stepan Gerasimov, Dec 08 2010
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STATUS
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approved
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