A246909 - OEIS

%I #7 Sep 08 2022 08:46:09

%S -1,2,1,28,15456,831376

%N a(n) = the smallest numbers k such that sigma(k+sigma(k)) = n* sigma(k) or -1 if no solution exists or has been found for n.

%C a(7) > 10^7 or -1.

%e Sequence of numbers k such that sigma(k+sigma(k)) = n* sigma(k)  for 1 <= n <= 6:

%e n = 2: 2, 3, 5, 11, 23, 29, 41, 53, 83, 89, 113, 131, 173, … (A246857).

%e n = 3: 1, 7, 26, 30, 42, 54, 69, 78, 84, 94, 102, 103, 114, … (A246910).

%e n = 4: 28, 66, 348, 496, 840, 920, 1320, 1416, 1602, 1770, … (A246911).

%e n = 5: 15456, 16920, 48576, 59520, 107160, 153360, 232596, … (A246912).

%e n = 6: 831376, 3944688, 16956576, 17843616, … (A246913).

%o (Magma) A246909:=func<n|exists(r){m: m in[1..1000000] | SumOfDivisors(m+SumOfDivisors(m))eq n*SumOfDivisors(m)}select r else-1>; [A246909(n): n in[1..10]]

%Y Cf. A000203, A246910, A246911, A246912, A246913, A246914.

%K sign

%O 1,2

%A _Jaroslav Krizek_, Sep 07 2014