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 A349659 #13 Feb 16 2025 08:34:02 %S A349659 0,0,3,4,5,9,7,4,3,15,11,13,13,21,23,20,17,9,19,39,31,33,23,13,30,39, %T A349659 30,53,29,69,31,52,47,51,47,13,37,57,55,39,41,93,43,81,68,69,47,77,56, %U A349659 90,71,95,53,90,71,53,79,87,59,93,61,93,94,116,83,141,67,123,95,141,71 %N A349659 Sum of the nonrefactorable divisors of n. %H A349659 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/RefactorableNumber.html">Refactorable Number</a> %F A349659 a(n) = A000203(n) - A335182(n). %F A349659 a(p) = p for odd primes p. - _Wesley Ivan Hurt_, Nov 28 2021 %t A349659 a[n_] := DivisorSum[n, # &, !Divisible[#, DivisorSigma[0, #]] &]; Array[a, 100] (* _Amiram Eldar_, Nov 24 2021 *) %o A349659 (PARI) a(n) = sumdiv(n, d, if (d%numdiv(d), d)); \\ _Michel Marcus_, Nov 24 2021 %Y A349659 Cf. A000203, A335182. %K A349659 nonn %O A349659 1,3 %A A349659 _Wesley Ivan Hurt_, Nov 23 2021