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 A010121 #67 Jul 26 2025 03:07:27
%S A010121 2,1,1,1,4,1,1,1,4,1,1,1,4,1,1,1,4,1,1,1,4,1,1,1,4,1,1,1,4,1,1,1,4,1,
%T A010121 1,1,4,1,1,1,4,1,1,1,4,1,1,1,4,1,1,1,4,1,1,1,4,1,1,1,4,1,1,1,4,1,1,1,
%U A010121 4,1,1,1,4,1,1,1,4,1,1,1,4
%N A010121 Continued fraction for sqrt(7).
%C A010121 This is a basic member of a family of 4-periodic multiplicative sequences with two parameters (c1,c2), defined for n >= 1 by a(n)=1 if n is odd, a(n)=c1 if n == 0 (mod 4) and a(n)=c2 if n == 2 (mod 4). Here, (c1,c2)=(4,1).
%C A010121 The Dirichlet generating function is (1+(c2-1)/2^s+(c1-c2)/4^s)*zeta(s).
%C A010121 Other members are A010123 with parameters (6,2), A010127 (8,3), A010130 (10,1), A010131 (10,2), A010132 (10,4), A010137 (12,5), A010146 (14,6), A089146 (4,8), A109008 (4,2), A112132 (7,3). If c1=c2, this reduces to the cases discussed in A040001. - _R. J. Mathar_, Feb 18 2011
%D A010121 James J. Tattersall, Elementary Number Theory in Nine Chapters, Cambridge University Press, 1999, page 276.
%H A010121 Harry J. Smith, <a href="/A010121/b010121.txt">Table of n, a(n) for n = 0..20000</a>
%H A010121 C. Elsner, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL14/Elsner/elsner9.html">Series of Error Terms for Rational Approximations of Irrational Numbers</a>, J. Int. Seq. 14 (2011) # 11.1.4, example 5.
%H A010121 G. Xiao, <a href="http://wims.unice.fr/~wims/en_tool~number~contfrac.en.html">Contfrac</a>
%H A010121 <a href="/index/Con#confC">Index entries for continued fractions for constants</a>
%H A010121 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,0,1).
%F A010121 From _R. J. Mathar_, Jun 17 2009: (Start)
%F A010121 G.f.: -(2*x^2+3*x+2)*(x^2-x+1)/((x-1)*(1+x)*(x^2+1)).
%F A010121 a(n) = a(n-4), n > 4. (End)
%F A010121 a(n) = (7 + 3*(-1)^n + 3*(-i)^n + 3*i^n)/4, n > 0, where i is the imaginary unit. - _Bruno Berselli_, Feb 18 2011
%e A010121 2.645751311064590590501615753... = A010465 = 2 + 1/(1 + 1/(1 + 1/(1 + 1/(4 + ...)))).
%t A010121 ContinuedFraction[Sqrt[7],300] (* _Vladimir Joseph Stephan Orlovsky_, Mar 04 2011 *)
%t A010121 CoefficientList[Series[(2 x^2 + 3 x + 2) (x^2 - x + 1) / ((1 - x) (1 + x) (x^2 + 1)), {x, 0, 100}], x] (* _Vincenzo Librandi_, Nov 26 2016 *)
%t A010121 PadRight[{2},120,{4,1,1,1}] (* _Harvey P. Dale_, Nov 30 2019 *)
%o A010121 (PARI) { allocatemem(932245000); default(realprecision, 13000); x=contfrac(sqrt(7)); for (n=0, 20000, write("b010121.txt", n, " ", x[n+1])); } \\ _Harry J. Smith_, Jun 01 2009
%Y A010121 Cf. A010465 (decimal expansion).
%K A010121 nonn,cofr,easy,mult
%O A010121 0,1
%A A010121 _N. J. A. Sloane_