This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A104543 #21 Apr 22 2020 18:50:16
%S A104543 1,2,3,2,2,3,1,2,4,3,1,3,3,2,4,4,1,3,3,2,4,4,4,5,1,5,4,5,4,5,1,2,4,5,
%T A104543 5,4,2,2,3,4,3,4,4,3,5,2,5,5,3,3,6,4,2,5,4,6,2,5,4,3,4,4,6,5,5,7,3,5,
%U A104543 5,3,4,6,5,8,3,5,5,5,2,4,4,3,4,5,3,7,6
%N A104543 Number of distinct prime divisors of 55...559 (with n 5s).
%H A104543 Amiram Eldar, <a href="/A104543/b104543.txt">Table of n, a(n) for n = 1..202</a>
%F A104543 a(n) = A001221((31+50*10^n)/9). - _R. J. Mathar_, Aug 24 2011
%e A104543 The number of distinct prime divisors of 59 is 1 (prime).
%e A104543 The number of distinct prime divisors of 559 is 2.
%e A104543 The number of distinct prime divisors of 5559 is 3.
%p A104543 A104543 := proc(n) local x ;x := [9,seq(5,k=1..n)] ; add(op(i,x)*10^(i-1),i=1..nops(x)) ; numtheory[factorset](%) ; nops(%) ; end proc: # _R. J. Mathar_, Aug 24 2011
%t A104543 Table[PrimeNu[(50*10^n + 31)/9], {n,0,50}] (* _G. C. Greubel_, May 10 2017 *)
%t A104543 Table[PrimeNu[FromDigits[PadLeft[{9},n,5]]],{n,2,90}] (* _Harvey P. Dale_, Apr 22 2020 *)
%Y A104543 Cf. A104484, A104483.
%K A104543 nonn,base
%O A104543 1,2
%A A104543 _Parthasarathy Nambi_, Apr 20 2005
%E A104543 More terms from _Amiram Eldar_, Jan 25 2020