A098771 - OEIS

%I #9 Mar 07 2018 19:09:00

%S 1,2,3,4,5,6,7,8,9,38,58,66,87,568,766,2799,6478,6785,26486,36685,

%T 36893,64788,66758,67586,69664,77686,276895,467683,474668,488973,

%U 548678,587667,658648,666369,668954,839666,939669,958968,968765,2386698

%N Numbers n such that sigma(n)=sigma(d_1)*sigma(d_2)*...*sigma(d_k) where d_1 d_2 ... d_k is the decimal expansion of n.

%C Number of terms of this sequence up to 15000000 is 72. Has this sequence at least one multi-digit prime term?

%e 97888745 is in the sequence because sigma(97888745) = 117936000 = sigma(9)*sigma(7)*sigma(8)*sigma(8)*sigma(8)*sigma(7)*sigma(4)*sigma(5).

%t Do[If[h=IntegerDigits[n];l=Length[h];!MemberQ[h, 0]&&DivisorSigma[ 1, n]==Product[DivisorSigma[1, h[[k]]], {k, l}], Print[n]] {n, 100000000}]

%t Select[Range[24*10^5],DivisorSigma[1,#]==Times@@DivisorSigma[1, IntegerDigits[ #]]&] (* _Harvey P. Dale_, Mar 07 2018 *)

%Y Cf. A098684, A098685.

%K base,nonn

%O 1,2

%A _Farideh Firoozbakht_, Oct 03 2004