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 A059866 #26 Nov 20 2023 08:18:45
%S A059866 2,4,2,8,2,12,2,20,2,12,2,164,2,40,2,40,2,1208,2,660,2,1304,2,3056,2,
%T A059866 2492,2,1080,2,13004,2,10232,2,11296,2,148736,2,56576,2,615482,2,
%U A059866 44448,2,64,2,2628524,2,28219952,2,139558,2,3067080,2,2683626,2,90740360,2,103050292,2
%N A059866 Period of the continued fraction for sqrt(2^n-1).
%H A059866 Chai Wah Wu, <a href="/A059866/b059866.txt">Table of n, a(n) for n = 2..78</a>
%e A059866 For n=7 and n=8 the continued fractions are [[11], [3, 1, 2, 2, 7, 11, 7, 2, 2, 1, 3, 22]] and [[15], [1, 30]] with periods 12 and 2, respectively.
%p A059866 with(numtheory): [seq(nops(cfrac(sqrt(2^k-1),'periodic','quotients')[2]),k=2..30)];
%t A059866 Table[Length@ Last@ ContinuedFraction[Sqrt[2^n - 1]], {n, 2, 56}] (* _Michael De Vlieger_, Mar 21 2015 *)
%Y A059866 Cf. A003285, A059926, A059927, A064932.
%K A059866 nonn
%O A059866 2,1
%A A059866 _Labos Elemer_, Feb 28 2001
%E A059866 Corrected and extended by _Naohiro Nomoto_, Nov 09 2001
%E A059866 a(57)-a(60) from _Daniel Suteu_, Jan 25 2019