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 A243984 #20 Aug 19 2014 01:01:12 %S A243984 1,3,0,1,6,8,8,9,9,18,12,12,14,24,15,25,18,35,20,36,28,36,24,36,31,42, %T A243984 36,50,30,63,32,57,44,54,36,75,38,60,52,66,42,92,44,78,69,72,48,100, %U A243984 57,93,68,92,54,116,72,114,76,90,60,125,62,96,84,121,84,140,68,120 %N A243984 Sum of non-twin divisors of n. %C A243984 See A243917 for definition of non-twin divisor. %H A243984 Jens Kruse Andersen, <a href="/A243984/b243984.txt">Table of n, a(n) for n = 1..10000</a> %F A243984 a(n) = A000203(n) - A243983(n). %e A243984 The divisors of 40 are 1, 2, 4, 5, 8, 10, 20, 40. Of these, 1, 5, 20, 40 are non-twin divisors. So a(40) = the sum of these divisors, which is 66. %p A243984 f:= proc(n) local d; d:= numtheory[divisors](n); convert(d minus map(`+`,d,2) minus map(`+`,d,-2),`+`) end proc: %p A243984 map(f, [$1..100]); # _Robert Israel_, Aug 17 2014 %t A243984 a243984[n_Integer] := Total[Select[Divisors[n], If[And[# <= 2 || Divisible[n, # - 2] == False, Divisible[n, # + 2] == False], True, False] &]]; a243984 /@ Range[68] (* _Michael De Vlieger_, Aug 17 2014 *) %o A243984 (PARI) %o A243984 a(n) = s=0; fordiv(n, d, if(!((d>2 && n%(d-2)==0) || (d<=n-2 && n%(d+2)==0)), s+=d)); s %o A243984 for(n=1, 200, print1(a(n), ", ")) \\ _Colin Barker_, Jun 29 2014 %Y A243984 Cf. A000203, A132882, A243917, A243983. %K A243984 nonn,easy %O A243984 1,2 %A A243984 _Juri-Stepan Gerasimov_, Jun 16 2014