A177024 - OEIS

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

(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