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a(n) is the A106490 index where n first occurs.
1

%I #10 Aug 25 2016 19:30:14

%S 1,2,4,12,36,144,720,3600,25200,176400,1587600,17463600,192099600,

%T 2497294800,32464832400,551902150800,9382336563600,178264394708400,

%U 3387023499459600,77901540487570800,1791735431214128400,44793385780353210000,1299008187630243090000

%N a(n) is the A106490 index where n first occurs.

%e 0, 1, 2, and 3 first appear in A106490 at positions 1, 2, 4, and 12, respectively.

%t a[n_] := a[n] = If[n == 1, 0, Sum[1 + a[i[[2]]], {i, FactorInteger[n]}]];

%t Flatten@ Table[FirstPosition[#, k_ /; k == n], {n, 0, 9}] &@ Table[a[n], {n, 10^6}] (* _Michael De Vlieger_, Aug 25 2016, Version 10, after _Jean-François Alcover_ at A106490 *)

%Y Cf. A106490.

%K nonn

%O 0,2

%A _Hans Havermann_, Aug 24 2016

%E More terms from _Alois P. Heinz_, Aug 25 2016