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 A309347 #16 Jul 30 2025 01:01:57
%S A309347 3,10,17,27,36,48,54,60,84,88,96,104,108,112,120,128,132,136,144,152,
%T A309347 156,168,176,180,184,192,200,204,208,216,224,228,232,240,248,252,256,
%U A309347 264,272,276,280,288,296,300,304,312,324,328,336,344,348,352,360,368
%N A309347 Sums of two refactorable numbers whose difference is refactorable.
%C A309347 Includes k+2 for k in A114617. - _Robert Israel_, Jul 29 2025
%H A309347 Robert Israel, <a href="/A309347/b309347.txt">Table of n, a(n) for n = 1..10000</a>
%H A309347 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/RefactorableNumber.html">Refactorable Number</a>
%p A309347 isref:= proc(n) option remember; n mod numtheory:-tau(n) = 0 end proc:
%p A309347 filter:= proc(n)
%p A309347 ormap(t -> isref(t) and isref(n-t) and isref(n-2*t), [$1 .. (n-1)/2])
%p A309347 end proc:
%p A309347 select(filter, [$1..1000]); # _Robert Israel_, Jul 29 2025
%t A309347 Flatten[Table[If[Sum[ (1 - Ceiling[(n - 2 i)/DivisorSigma[0, n - 2 i]] + Floor[(n - 2 i)/DivisorSigma[0, n - 2 i]]) (1 - Ceiling[i/DivisorSigma[0, i]] + Floor[i/DivisorSigma[0, i]]) (1 - Ceiling[(n - i)/DivisorSigma[0, n - i]] + Floor[(n - i)/DivisorSigma[0, n - i]]), {i, Floor[(n - 1)/2]}] > 0, n, {}], {n, 400}]]
%Y A309347 Cf. A033950, A114617.
%K A309347 nonn
%O A309347 1,1
%A A309347 _Wesley Ivan Hurt_, Jul 24 2019