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 A379590 #32 Jan 11 2025 19:08:34 %S A379590 0,1,1,1,1,2,1,1,1,2,0,2,1,2,2,1,1,2,1,2,2,1,0,2,1,2,1,2,0,3,1,1,1,2, %T A379590 2,2,0,2,2,2,0,3,0,1,2,1,0,2,1,2,2,2,0,2,1,2,2,1,0,3,1,2,2,1,2,2,0,2, %U A379590 1,3,0,2,0,1,2,2,1,3,0,2,1,1,0,3,2,1,1,1,1,3,2,1,2,1,2,2,0,2,1,2 %N A379590 a(n) is the number of prime divisors d of n such that 2^d - 1 is prime. %C A379590 Number of divisors of n that belong to A000043. %t A379590 a[n_] := DivisorSum[n, 1 &, PrimeQ[2^# - 1] &]; Array[a, 100] (* _Amiram Eldar_, Dec 27 2024 *) %o A379590 (Magma) [#[d: d in Divisors(n) | IsPrime(2^d-1)]: n in [1..100]]; %o A379590 (PARI) a(n) = sumdiv(n, d, isprime(d) && ispseudoprime(2^d-1)); \\ _Michel Marcus_, Dec 28 2024 %Y A379590 Cf. A000005, A000043, A054723. %K A379590 nonn %O A379590 1,6 %A A379590 _Juri-Stepan Gerasimov_, Dec 26 2024