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 A105259 #23 Apr 08 2020 00:21:54
%S A105259 0,2,1,2,1,3,1,4,2,3,3,3,3,4,3,4,3,6,2,4,4,3,3,5,4,7,4,6,2,6,3,6,1,2,
%T A105259 3,5,3,10,4,7,5,4,6,7,1,7,2,6,3,5,5,6,4,6,2,8,4,7,3,5,4,11,2,7,5,8,6,
%U A105259 5,5,7,2,8,4,7,5,6,4,6,5,9,3,9,4,7,2,9,4
%N A105259 Number of distinct prime divisors of 99..91 (with n 9's).
%H A105259 Amiram Eldar, <a href="/A105259/b105259.txt">Table of n, a(n) for n = 0..229</a>
%F A105259 a(n) = A001221(A170955(n+1)). - _R. J. Mathar_, Aug 24 2011
%e A105259 If n=1, then the number of distinct prime divisors of 91 is 2.
%e A105259 If n=2, then the number of distinct prime divisors of 991 is 1 (a prime).
%e A105259 If n=3, then the number of distinct prime divisors of 9991 is 2.
%p A105259 A105259 := proc(n) local x ;x := [1,seq(9,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 A105259 Table[PrimeNu[10^(n + 1) - 9], {n, 0, 50}] (* _G. C. Greubel_, May 10 2017 *)
%Y A105259 Cf. A001221, A093177, A170955.
%K A105259 nonn,base
%O A105259 0,2
%A A105259 _Parthasarathy Nambi_, Apr 14 2005
%E A105259 More terms from _Amiram Eldar_, Jan 24 2020