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 A380288 #11 Feb 03 2025 23:05:35 %S A380288 1,1,1,2,1,2,1,3,2,2,2,4,1,2,2,4,1,4,1,4,2,3,2,6,2,2,3,4,2,5,1,5,3,2, %T A380288 2,7,2,2,2,6,2,5,2,5,4,3,2,8,2,4,2,4,2,6,3,6,2,3,2,9,1,2,4,6,2,6,2,4, %U A380288 3,5,2,10,2,3,4,4,3,5,2,8,4,3,2,9,2,3,3,7,1,9,2,5,2,3,2,10,2,4,5,7 %N A380288 a(n) is the number of divisors d of n such that 2^d - 1 is not prime. %F A380288 a(n) = A000005(n) - A379590(n). %t A380288 a[n_] := DivisorSum[n, 1 &, !PrimeQ[2^# - 1] &]; Array[a, 100] (* _Amiram Eldar_, Jan 19 2025 *) %o A380288 (Magma) [#[d: d in Divisors(n) | not IsPrime(2^d-1)]: n in [1..100]]; %o A380288 (PARI) a(n) = sumdiv(n, d, !isprime(2^d - 1)); \\ _Michel Marcus_, Jan 19 2025 %Y A380288 Cf. A000005, A000043, A379590. %K A380288 nonn %O A380288 1,4 %A A380288 _Juri-Stepan Gerasimov_, Jan 18 2025