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 A027556 #31 Apr 28 2019 14:38:54 %S A027556 0,0,0,2,6,18,42,98,210,450,930,1922,3906,7938,16002,32258,64770, %T A027556 130050,260610,522242,1045506,2093058,4188162,8380418,16764930, %U A027556 33538050,67084290,134184962,268386306,536805378,1073643522 %N A027556 Unbalanced strings of length n. %C A027556 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 triangle with side length one. E.g., mark the vertices of the triangle as 1, 2 and 3. Having a yardstick with n = 3 sticks there are two ways of folding to form an equilateral triangle: 1-2-3-1 and 1-3-2-1. For n = 4 there are six ways of folding: 1-2-3-1-2 or 3; 1-3-2-1-2 or 3; 1-2-1-3-2 and 1-3-1-2-3. - _Frank Hollstein_, Apr 23 2019 %H A027556 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (3,0,-6,4). %F A027556 a(n) = 2^n - A027383(n). %F A027556 G.f.: 2*x^2/((1-x)*(1-2*x)(1-2*x^2)). %Y A027556 Cf. A000079 (2^n), A027383. %K A027556 nonn,easy %O A027556 0,4 %A A027556 _R. K. Guy_, _David Callan_