A309346 - OEIS

%I #9 Apr 20 2020 18:56:40

%S 2,3,4,9,10,11,13,14,16,17,18,19,20,21,24,25,26,27,30,32,33,36,37,38,

%T 41,42,44,45,48,49,52,54,57,58,60,61,62,64,65,68,69,72,73,74,76,78,80,

%U 81,82,84,85,86,88,89,90,92,93,96,97,98,100,102,104,105

%N Sums of two refactorable numbers.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/RefactorableNumber.html">Refactorable Number</a>

%e 3 is in the sequence since 3 = 1 + 2 and both 1 and 2 are refactorable (since d(1) = 1 | 1 and d(2) = 2 | 2).

%e 4 is in the sequence since 4 = 2 + 2 and 2 is refactorable (since d(2) | 2).

%t Flatten[Table[If[Sum[ (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/2]}] > 0, n, {}], {n, 100}]]

%Y Cf. A033950.

%K nonn

%O 1,1

%A _Wesley Ivan Hurt_, Jul 24 2019