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 A228870 #16 Dec 29 2016 16:59:13
%S A228870 6,12,18,20,24,30,36,40,42,48,54,60,66,72,78,80,84,90,96,100,102,108,
%T A228870 110,114,120,126,132,138,140,144,150,156,160,162,168,174,180,186,192,
%U A228870 198,200,204,210,216,220,222,228,234,240,246,252,258,260,264,270,272
%N A228870 Numbers n such that 2 * (1^n + 2^n + 3^n + ... + n^n) is not 0 (mod n).
%C A228870 These are the numbers not appearing in A228869; the even numbers not in A226872.
%C A228870 Also, positive integers n such that there exists an odd prime divisor p of n such that (p-1) also divides n (cf. A124240). - _Max Alekseyev_, Sep 07 2013
%C A228870 This sequence agrees with A088723 for many terms, but they are different.
%C A228870 If n is in the sequence, then so are the multiples of n. See A280187 for primitive members of this sequence. - _Charles R Greathouse IV_, Dec 28 2016
%H A228870 Vincenzo Librandi, <a href="/A228870/b228870.txt">Table of n, a(n) for n = 1..1000</a>
%t A228870 Select[Range[100], Mod[2*Sum[PowerMod[k, #, #], {k, #}], #] > 0 &]
%o A228870 (PARI) is(n)=my(f=factor(n)[,1]); for(i=1,#f, if(n%(f[i]-1)==0 && f[i]>2, return(1))); 0 \\ _Charles R Greathouse IV_, Dec 28 2016
%Y A228870 Cf. A228869, A226872.
%K A228870 nonn
%O A228870 1,1
%A A228870 _T. D. Noe_, Sep 06 2013