cp's OEIS Frontend

A048995 Numbers that are not the sum of the nontrivial factors (excluding 1 and n) of some natural number.

%I A048995 #24 Nov 05 2019 06:51:11
%S A048995 1,4,51,87,95,119,123,145,161,187,205,209,215,237,245,247,261,267,275,
%T A048995 287,289,291,303,305,321,323,325,335,341,371,405,407,425,429,447,471,
%U A048995 473,497,515,517,519,529,539,551,555,561,575,583,611,623,625,627,657
%N A048995 Numbers that are not the sum of the nontrivial factors (excluding 1 and n) of some natural number.
%H A048995 Amiram Eldar, <a href="/A048995/b048995.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Donovan Johnson)
%t A048995 a048995[n_Integer] := Module[{t = Table[i, {i, n}], a048050, k},
%t A048995   a048050[m_] := Plus @@ Take[Divisors[m], {2, -2}];
%t A048995   Do[
%t A048995    If[a048050[k] == 0 || a048050[k] > n, Null, t[[a048050[k]]] = 0],
%t A048995    {k, 2, n^2}];
%t A048995   Drop[DeleteDuplicates[t], {2}]
%t A048995 ]; a048995[660] (* _Michael De Vlieger_, Nov 30 2014 *)
%o A048995 (PARI) mx=8479; v=vector(mx); for(i=2, mx^2, ch=sigma(i)-i-1; if(ch<=mx, if(ch>0, v[ch]=1))); c=0; for(i=1, mx, if(v[i]==0, c++; write("b048995.txt", c " " i))) /* _Donovan Johnson_, Feb 11 2013 */
%Y A048995 Complement of A048050.
%Y A048995 Cf. A005114 (the same property with the sum of proper divisors) and A007369 (the same property with the sum of all divisors).
%K A048995 nonn,nice
%O A048995 1,2
%A A048995 Bill Taylor (W.Taylor(AT)math.canterbury.ac.nz)
%E A048995 Corrected and extended by _Jud McCranie_