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 A104518 #17 Jan 25 2020 08:01:01
%S A104518 1,2,2,2,2,2,1,3,2,2,3,4,3,3,4,2,2,4,3,3,5,2,2,6,1,3,4,5,7,4,4,3,6,4,
%T A104518 4,7,5,6,4,3,3,5,2,4,6,4,5,6,4,4,4,5,4,4,4,5,4,3,2,5,3,3,3,3,1,5,7,6,
%U A104518 2,3,5,3,1,7,2,4,3,8,4,6,3,3,3,6,5,3,5
%N A104518 Number of distinct prime divisors of 55...3 (with n 5s).
%H A104518 Amiram Eldar, <a href="/A104518/b104518.txt">Table of n, a(n) for n = 1..199</a>
%F A104518 a(n) = A001221(A173802(n+1)). - _R. J. Mathar_, Aug 24 2011
%e A104518 The number of distinct prime divisors of 53 is 1 (prime) which is the first term in the sequence.
%e A104518 The number of distinct prime divisors of 553 is 2 which is the second term in the sequence.
%e A104518 The number of distinct prime divisors of 5553 is 2 which is the third term in the sequence.
%p A104518 A104518 := proc(n) local x ;x := [3,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 A104518 Table[PrimeNu[(50*10^n - 23)/9], {n, 0, 50}] (* _G. C. Greubel_, May 06 2017 *)
%Y A104518 Cf. A001221, A173802.
%K A104518 nonn,base
%O A104518 1,2
%A A104518 _Parthasarathy Nambi_, Apr 19 2005
%E A104518 More terms from _Amiram Eldar_, Jan 25 2020