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 A104517 #20 Sep 08 2022 08:45:17
%S A104517 2,2,3,2,2,2,3,2,5,4,1,1,3,2,5,3,4,2,4,5,4,5,3,2,3,3,3,5,3,4,6,4,4,2,
%T A104517 4,4,3,3,5,2,2,3,2,3,7,4,3,2,5,4,4,4,6,4,8,5,3,4,7,3,2,3,4,4,5,5,5,5,
%U A104517 6,3,5,4,2,4,4,6,4,3,2,2,6,3,5,7,5,3,6,3,4,6,7,7
%N A104517 Number of distinct prime divisors of 55...1 (with n 5s).
%C A104517 Number of distinct prime factors of (10^(n + 1) - 1)*5/9 - 4. - _Stefan Steinerberger_, Mar 06 2006
%H A104517 Amiram Eldar, <a href="/A104517/b104517.txt">Table of n, a(n) for n = 1..200</a>
%F A104517 a(n) = A001221(A173804(n+1)). - _Amiram Eldar_, Jan 24 2020
%e A104517 The number of distinct prime divisors of 51 is 2 which is the first term in the sequence.
%e A104517 The number of distinct prime divisors of 551 is 2 which is the second term in the sequence.
%e A104517 The number of distinct prime divisors of 5551 is 3 which is the third term in the sequence.
%p A104517 f:= n -> nops(numtheory:-factorset( (10^(n + 1) - 1)*5/9 - 4)):
%p A104517 map(f, [$1..92]); # _Robert Israel_, Mar 08 2018
%t A104517 Table[Length[FactorInteger[(10^(n + 1) - 1)*5/9 - 4]], {n, 1, 50}] (* _Stefan Steinerberger_, Mar 06 2006 *)
%o A104517 (Magma) [#PrimeDivisors((10^(n+1)-1)*5 div 9-4): n in [1..80]]; // _Vincenzo Librandi_, Mar 09 2018
%o A104517 (PARI) a(n) = omega((10^(n + 1) - 1)*5/9 - 4); \\ _Michel Marcus_, Mar 09 2018
%Y A104517 Cf. A001221, A056684 (a(n)=1), A104484, A173804.
%K A104517 nonn,base
%O A104517 1,1
%A A104517 _Parthasarathy Nambi_, Apr 19 2005
%E A104517 More terms from _Stefan Steinerberger_, Mar 06 2006
%E A104517 a(51)-a(92), and offset corrected, by _Robert Israel_, Mar 08 2018