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 A171526 #27 Mar 22 2025 17:26:35
%S A171526 1,1,1,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,
%T A171526 27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,
%U A171526 50
%N A171526 Denominator of (n-th noncomposite/n).
%C A171526 Denominator of (A008578(n)/n).
%C A171526 From _Enrique Navarrete_, Mar 15 2025: (Start)
%C A171526 a(n+1) is the number of binary strings of length n with at most one 0 and either zero or at least two 1s. For example, for n=2, a(3)=1 since the string is 11; for n=3, a(4)=4 since the strings are 111, 011, 101, 110.
%C A171526 a(n+1) is also the number of ordered set partitions of an n-set into 2 sets such that the first set is empty or has one element and the second set is empty or has at least two elements. (End)
%F A171526 a(1) = a(2) = a(3) = 1, a(n) = n for n >= 4.
%F A171526 From _Enrique Navarrete_, Mar 15 2025: (Start)
%F A171526 G.f.: x*(1-x+3*x^3-2*x^4)/(1-x)^2.
%F A171526 E.g.f.: x*exp(x)-x^2/2-x^3/3. (End)
%t A171526 Module[{nn=300,c,len},c=Select[Range[nn],!CompositeQ[#]&];len=Length[ c]; #[[1]]/#[[2]]&/@Thread[{c,Range[len]}]]//Denominator (* or *) Join[{1,1,1}, Range[ 4,60]] (* _Harvey P. Dale_, Feb 08 2020 *)
%Y A171526 Cf. A008578, A065475.
%K A171526 nonn,easy
%O A171526 1,4
%A A171526 _Jaroslav Krizek_, Dec 11 2009
%E A171526 Corrected by _Jaroslav Krizek_, Dec 16 2009