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 A190290 #8 Aug 03 2020 02:33:26
%S A190290 2,5,2,7,5,2,5,2,3,1,6,5,1,9,4,6,6,6,8,8,6,2,6,8,2,3,9,7,9,0,9,3,3,6,
%T A190290 1,6,2,9,9,4,8,1,8,8,5,8,9,2,2,6,5,7,3,0,0,8,6,9,0,8,0,7,0,7,9,6,8,9,
%U A190290 5,6,1,4,1,8,4,9,2,5,6,9,6,2,2,0,1,4,5,3,8,5,3,1,6,4,4,8,1,6,7,7,5,5,9,2,0,0,3,0,1,7,9,9,1,9,5,2,4,6,9,5
%N A190290 Decimal expansion of (3+sqrt(21))/3.
%C A190290 The rectangle R whose shape (i.e., length/width) is (3+sqrt(21))/3, can be partitioned into rectangles of shapes 3/2 and 2 in a manner that matches the periodic continued fraction [2, 3/2, 2, 3/2, ...]. R can also be partitioned into squares so as to match the periodic continued fraction [2,1,1,8,1,1,2,1,1,8,1,1,2,,...]. For details, see A188635.
%F A190290 Equals 1 + Sum_{k>=0} binomial(2*k,k)/7^k. - _Amiram Eldar_, Aug 03 2020
%e A190290 2.527525231651946668862682397909336162995...
%t A190290 FromContinuedFraction[{2, 3/2, {2, 3/2}}]
%t A190290 ContinuedFraction[%, 100] (* [2,1,1,8,1,1,2,... *)
%t A190290 RealDigits[N[%%, 120]] (* A190290 *)
%t A190290 N[%%%, 40]
%Y A190290 Cf. A188635, A190289.
%K A190290 nonn,cons
%O A190290 1,1
%A A190290 _Clark Kimberling_, May 07 2011