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 A027558 #20 May 02 2019 09:44:42 %S A027558 0,0,0,0,2,6,20,50,126,288,654,1422,3068,6474,13574,28112,57942, %T A027558 118566,241788,490914,994334,2008512,4050398,8154014,16396668, %U A027558 32935194,66104982,132586416,265792070,532579830,1066790972 %N A027558 Number of 3-unbalanced strings of length n (= 2^n - A027557(n)). %C A027558 a(n) is also the number of different ways to fold down a folding yardstick with n sticks (stick length is one) to form an equilateral tetragon (parallelogram) with side length one. - _Frank Hollstein_, May 01 2019 %H A027558 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (3,1,-8,2,4). %F A027558 a(n) = 2*A005674(n). %F A027558 G.f.: -2*x^4/((2*x-1)*(2*x^2-1)*(x^2+x-1)). [Maksym Voznyy (voznyy(AT)mail.ru), Aug 12 2009] %K A027558 nonn %O A027558 0,5 %A A027558 _R. K. Guy_, _David Callan_