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 A104564 #19 Apr 10 2020 01:42:49
%S A104564 1,2,2,2,3,2,2,2,2,3,3,1,3,4,2,2,3,2,1,4,3,1,3,3,2,3,4,2,7,1,5,6,3,3,
%T A104564 4,3,4,5,5,3,5,4,2,3,3,6,2,2,6,5,4,3,4,4,6,2,6,5,4,4,4,5,3,3,5,2,6,3,
%U A104564 5,5,4,6,4,6,5,3,3,4,4,4,4,6,4,3,6,5,3
%N A104564 Number of distinct prime divisors of 77...771 (with n 7's).
%C A104564 Also number of distinct prime factors of (10^(n + 1) - 1)*7/9 - 6. - _Stefan Steinerberger_, Mar 01 2006
%H A104564 Amiram Eldar, <a href="/A104564/b104564.txt">Table of n, a(n) for n = 1..231</a>
%F A104564 a(n) = A001221(A173806(n+1)). - _Amiram Eldar_, Jan 24 2020
%e A104564 The number of distinct prime divisors of 71 is 1 (prime).
%e A104564 The number of distinct prime divisors of 771 is 2.
%e A104564 The number of distinct prime divisors of 7771 is 2.
%t A104564 Table[Length[FactorInteger[(10^(n + 1) - 1)*7/9 - 6]], {n, 1, 50}] (* _Stefan Steinerberger_, Mar 01 2006 *)
%t A104564 PrimeNu/@(FromDigits/@Table[PadLeft[{1},n,7],{n,2,55}]) (* _Harvey P. Dale_, Apr 22 2011 *)
%Y A104564 Cf. A001221, A104484, A104483, A173806.
%Y A104564 Cf. A104484 (3 instead of 7), A104659 (4 instead of 7), A104517 (5 instead of 7), A104890 (6 instead of 7), A105972 (8 instead of 7), A105259 (9 instead of 7).
%K A104564 nonn,base
%O A104564 1,2
%A A104564 _Parthasarathy Nambi_, Apr 20 2005
%E A104564 More terms from _Stefan Steinerberger_, Mar 01 2006
%E A104564 Offset corrected and more terms added by _Amiram Eldar_, Jan 24 2020