This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A177024 #16 Jul 12 2022 08:30:23 %S A177024 15,21,24,33,39,40,51,57,69,87,93,111,123,129,141,154,159,177,183,201, %T A177024 213,219,237,249,267,291,303,309,321,327,339,381,393,411,417,447,453, %U A177024 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 %N A177024 Numbers k such that 2^(k-1) mod k = number of divisors of k. %H A177024 Amiram Eldar, <a href="/A177024/b177024.txt">Table of n, a(n) for n = 1..10000</a> %F A177024 A062173(a(n)) = A000005(a(n)). %t A177024 Select[Range[1000], Mod[2^(# - 1), #] == Length[Divisors[#]] &] %t A177024 Select[Range[1000],PowerMod[2,#-1,#]==Length[Divisors[#]]&] (* _Harvey P. Dale_, Nov 19 2015 *) %t A177024 Select[Range[1000], PowerMod[2, #-1, #] == DivisorSigma[0, #] &] (* _Amiram Eldar_, Jul 12 2022 *) %Y A177024 Cf. A000005, A062173, A176175. %K A177024 nonn %O A177024 1,1 %A A177024 _Juri-Stepan Gerasimov_, Dec 08 2010