OFFSET
1,3
FORMULA
a(n) = n * Sum_{i=1..floor((n-1)/2)} floor(n/i) - floor((n-1)/i).
a(n) = n * A023645(n). - Robert G. Wilson v, Dec 24 2017
EXAMPLE
From Wesley Ivan Hurt, Feb 21 2018: (Start)
a(5) = 5; there is one partition of 5 into two distinct parts such that the smaller part divides the larger, namely (4,1), so the sum of the parts is then 4 + 1 = 5.
a(6) = 12; the partitions of 6 into two distinct parts such that the smaller part divides the larger are (5,1) and (4,2), and the sum of the parts is then 5 + 1 + 4 + 2 = 12.
a(7) = 7; there is one partition of 7 into two distinct parts such that the smaller part divides the larger, namely (6,1), so the sum of the parts is 6 + 1 = 7.
a(8) = 16; there are two partitions of 8 into 2 distinct parts such that the smaller divides the larger, namely (7,1) and (6,2). The sum of the parts is then 7 + 1 + 6 + 2 = 16.
(End)
MATHEMATICA
Table[n*Sum[(Floor[n/i] - Floor[(n - 1)/i]), {i, Floor[(n - 1)/2]}], {n, 100}]
f[n_] := n*Length[Select[Divisors@n, 2 # < n &]]; Array[f, 61] (* or *)
f[n_] := Block[{t = DivisorSigma[0, n]}, n*If[OddQ@ n, t -1, t -2]]; Array[f, 61] (* Robert G. Wilson v, Dec 24 2017 *)
PROG
(PARI) a(n) = n*sum(i=1, floor((n-1)/2), floor(n/i) - floor((n-1)/i)) \\ Iain Fox, Dec 22 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wesley Ivan Hurt, Dec 22 2017
STATUS
approved