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A177024 Numbers k such that 2^(k-1) mod k = number of divisors of k. 1
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

FORMULA

A062173(a(n)) = A000005(a(n)).

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 *)

CROSSREFS

Cf. A000005, A062173, A176175.

Sequence in context: A343821 A294171 A306102 * A325037 A154545 A156063

Adjacent sequences:  A177021 A177022 A177023 * A177025 A177026 A177027

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, Dec 08 2010

STATUS

approved

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Last modified August 9 15:33 EDT 2022. Contains 356026 sequences. (Running on oeis4.)