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 A386390 #17 Aug 21 2025 13:42:27 %S A386390 2,6,66,225,8646,101025,149497986,20412000225 %N A386390 Numbers k such that k-1 | sigma+(k) where sigma+ is A107758. %t A386390 a107758[n_]:=DivisorSum[n, DivisorSigma[1, #] &, CoprimeQ[n/#, #] &];Select[Range[2,10^6],Divisible[a107758[#],#-1]&] (* _James C. McMahon_, Aug 21 2025 *) %o A386390 (PARI) isok(k) = if (k>1, !(sumdiv(k, d, if(gcd(k/d, d) == 1, sigma(d))) % (k-1))); %Y A386390 Cf. A107758, A055708. %K A386390 nonn,more,new %O A386390 1,1 %A A386390 _Michel Marcus_, Aug 20 2025 %E A386390 a(8) from _Vincenzo Librandi_, Aug 21 2025