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 A105561 #19 Apr 17 2020 10:41:10
%S A105561 1,2,2,2,2,3,2,2,2,3,2,3,2,3,3,2,2,3,2,3,3,3,2,3,2,3,2,3,2,5,2,2,3,3,
%T A105561 3,3,2,3,3,3,2,5,2,3,3,3,2,3,2,3,3,3,2,3,3,3,3,3,2,5,2,3,3,2,3,5,2,3,
%U A105561 3,5,2,3,2,3,3,3,3,5,2,3,2,3,2,5,3,3,3,3,2,5,3,3,3,3,3,3,2,3,3,3,2,5,2,3,5,3,2,3,2,5,3,3,2,5,3,3,3,3,3,5
%N A105561 a(n) is the m-th prime, where m is the number of distinct prime factors of n (A001221), a(1) = 1.
%C A105561 Term a(1) = 1 prepended to match with the definition of A105560. - _Antti Karttunen_, May 25 2017
%H A105561 Antti Karttunen, <a href="/A105561/b105561.txt">Table of n, a(n) for n = 1..10000</a>
%e A105561 Let n = 6; 6 has 2 different prime factors, therefore a(6) = 3, the second prime.
%t A105561 Table[Prime[Length[FactorInteger[n]]], {n, 2, 84}]
%t A105561 Prime[PrimeNu[Range[2,90]]] (* _Harvey P. Dale_, Oct 02 2013 *)
%o A105561 (PARI) A105561(n) = if(1==n,n,prime(omega(n))); \\ [After the original Pari-program given here.] - _Antti Karttunen_, May 25 2017
%o A105561 (Python)
%o A105561 from sympy import prime, primefactors
%o A105561 def a(n): return 1 if n==1 else prime(len(primefactors(n))) # _Indranil Ghosh_, May 25 2017
%Y A105561 Cf. A000040, A001221, A105555, A105560.
%K A105561 easy,nonn
%O A105561 1,2
%A A105561 _Cino Hilliard_, May 03 2005
%E A105561 Edited by _Stefan Steinerberger_, Jun 15 2007
%E A105561 Term a(1) = 1 prepended (correcting also the indexing of the rest of terms), and data section extended to 120 terms by _Antti Karttunen_, May 25 2017