%I #13 Jan 06 2024 22:35:49

%S 0,0,1,0,1,0,1,0,2,2,3,2,4,3,4,2,4,2,5,4,5,4,5,2,6,5,5,5,6,5,6,4,6,5,

%T 6,2,7,6,7,5,8,7,8,7,7,7,8,4,8,7,8,7,8,5,8,6,9,8,9,7,10,9,9,8,10,9,10,

%U 9,10,9,10,3,11,10,11,10,11,10,11,8,11,11,12,10

%N Number of refactorable numbers less than n that do not divide n.

%F a(n) = Sum_{k=1..n} c(k) * (ceiling(n/k) - floor(n/k)), where c = A336040.

%e a(15) = 4 since there are 4 refactorable numbers that are less than 15 and do not divide 15, namely: 2, 8, 9, 12.

%t Table[Sum[(1 - Ceiling[k/DivisorSigma[0, k]] + Floor[k/DivisorSigma[0, k]]) (Ceiling[n/k] - Floor[n/k]), {k, n}], {n, 100}]

%Y Cf. A033950, A336040, A368817.

%K nonn,easy

%O 1,9

%A _Wesley Ivan Hurt_, Jan 06 2024