OFFSET
1,2
COMMENTS
A prime number has no nontrivial divisors so their sum is = 0. That's why we take only composite numbers.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..139
FORMULA
Obviously a(n) = A238895(n)-1.
EXAMPLE
For 1, there are no numbers.
For 2, there is 1 number: 4.
For 5, there are 2 numbers: 6 and 25.
For 20, there are 3 numbers: 18, 51, 91.
MATHEMATICA
ch[1] = 0; ch[n_] := DivisorSigma[1, n] - n - 1; m = 300; v = Table[0, {m}]; Do[c = ch[k]; If[1 <= c <= m, v[[c]]++], {k, 1, m^2}]; s = {}; vm = -1; Do[If[v[[k]] > vm, vm = v[[k]]; AppendTo[s, k]], {k, 1, m}]; s (* Amiram Eldar, Nov 05 2019 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Daniel Lignon, Nov 22 2014
STATUS
approved