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 A124274 #16 Jul 25 2025 06:56:09
%S A124274 1,9,15,121
%N A124274 Nonprime numbers k that divide A124271(k) = Sum_{i=1..k} (prime(i)^k - 1) / (prime(i) - 1).
%C A124274 The next term if it exists is greater than 1000.
%C A124274 Note that a(1) = 1, a(2) = 3^2 and a(4) = 11^2 are perfect squares.
%C A124274 a(5) > 10^4, if it exists. - _Amiram Eldar_, Jul 25 2025
%e A124274 9 is a term because 9 divides A124271(9) = 108336139407.
%t A124274 s={};Do[If[!PrimeQ[k],If[Divisible[Sum[(Prime[i]^k-1)/(Prime[i]-1),{i,k}],k],AppendTo[s,k]]],{k,10^3}];s (* _James C. McMahon_, Dec 10 2024 *)
%Y A124274 Cf. A124271, A124273 (primes p that divide A124271(p)).
%K A124274 bref,hard,more,nonn
%O A124274 1,2
%A A124274 _Alexander Adamchuk_, Oct 23 2006