A349659 - OEIS

%I #13 Feb 16 2025 08:34:02

%S 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 30,53,29,69,31,52,47,51,47,13,37,57,55,39,41,93,43,81,68,69,47,77,56,

%U 90,71,95,53,90,71,53,79,87,59,93,61,93,94,116,83,141,67,123,95,141,71

%N Sum of the nonrefactorable divisors of n.

%H Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/RefactorableNumber.html">Refactorable Number</a>

%F a(n) = A000203(n) - A335182(n).

%F a(p) = p for odd primes p. - _Wesley Ivan Hurt_, Nov 28 2021

%t a[n_] := DivisorSum[n, # &, !Divisible[#, DivisorSigma[0, #]] &]; Array[a, 100] (* _Amiram Eldar_, Nov 24 2021 *)

%o (PARI) a(n) = sumdiv(n, d, if (d%numdiv(d), d)); \\ _Michel Marcus_, Nov 24 2021

%Y Cf. A000203, A335182.

%K nonn,changed

%O 1,3

%A _Wesley Ivan Hurt_, Nov 23 2021