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 A181301 #9 Jul 24 2022 14:12:47
%S A181301 1,2,6,20,64,206,662,2128,6840,21986,70670,227156,730152,2346942,
%T A181301 7543822,24248256,77941648,250529378,805281526,2588432308,8320049072,
%U A181301 26743297998,85961510758,276307781200,888141556360,2854770939522
%N A181301 Number of 2-compositions of n having no column with equal entries. A 2-composition of n is a nonnegative matrix with two rows, such that each column has at least one nonzero entry and whose entries sum up to n.
%C A181301 a(n)=A181299(n,0).
%D A181301 G. Castiglione, A. Frosini, E. Munarini, A. Restivo and S. Rinaldi, Combinatorial aspects of L-convex polyominoes, European Journal of Combinatorics, 28, 2007, 1724-1741.
%F A181301 G.f. = (1+z)(1-z)^2/(1-3z-z^2+z^3).
%F A181301 a(n) = Sum_{k, 0<=k<=n} A060086(n,k)*2^k. - _Philippe Deléham_, Feb 24 2012
%F A181301 a(n) = 2*A033505(n-1), n>0. - _R. J. Mathar_, Jul 24 2022
%p A181301 g := (1+z)*(1-z)^2/(1-3*z-z^2+z^3): gser := series(g, z = 0, 30): seq(coeff(gser, z, n), n = 0 .. 27);
%Y A181301 Cf. A181299.
%K A181301 nonn,easy
%O A181301 0,2
%A A181301 _Emeric Deutsch_, Oct 12 2010