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 A349658 #18 Feb 16 2025 08:34:02 %S A349658 0,0,1,1,1,2,1,1,1,2,1,3,1,2,3,2,1,2,1,4,3,2,1,3,2,2,2,4,1,6,1,3,3,2, %T A349658 3,3,1,2,3,4,1,6,1,4,4,2,1,5,2,4,3,4,1,4,3,4,3,2,1,8,1,2,4,4,3,6,1,4, %U A349658 3,6,1,3,1,2,5,4,3,6,1,5,3,2,1,8,3,2,3,4,1,8,3,4 %N A349658 Number of nonrefactorable divisors of n. %H A349658 Antti Karttunen, <a href="/A349658/b349658.txt">Table of n, a(n) for n = 1..20000</a> %H A349658 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/RefactorableNumber.html">Refactorable Number</a> %F A349658 a(n) = A000005(n) - A336041(n). %F A349658 a(n) = A349322(n) - A335182(n). - _Antti Karttunen_, Nov 24 2021 %F A349658 a(p) = 1 for odd primes p. - _Wesley Ivan Hurt_, Nov 28 2021 %t A349658 a[n_] := DivisorSum[n, 1 &, !Divisible[#, DivisorSigma[0, #]] &]; Array[a, 100] (* _Amiram Eldar_, Nov 24 2021 *) %o A349658 (PARI) a(n) = sumdiv(n, d, d%numdiv(d) != 0); \\ _Michel Marcus_, Nov 24 2021 %Y A349658 Cf. A000005, A033950, A336041, A335182, A349322. %K A349658 nonn %O A349658 1,6 %A A349658 _Wesley Ivan Hurt_, Nov 23 2021