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 A274217 #7 Feb 06 2017 12:44:09
%S A274217 0,4,12,30,72,166,376,837,1844,4025,8723,18789,40263,85892,182515,
%T A274217 386488,815890,1717604,3606854,7556995,15800562,32974263,68694801,
%U A274217 142882147,296749381,615468930,1274878193,2637637046,5451035408,11253593442,23210232132,47826554753
%N A274217 Sum of the site-perimeters of all bargraphs of area n.
%C A274217 A bargraph is a polyomino whose bottom is a segment of the nonnegative x-axis and whose upper part is a lattice path starting at (0,0) and ending with its first return to the x-axis using steps U=(0,1), D=(0,-1) and H=(1,0), where UD and DU are not allowed.
%C A274217 The site-perimeter of a polyomino is the number of exterior cells having a common edge with at least one polyomino cell.
%H A274217 M. Bousquet-Mélou and A. Rechnitzer, <a href="http://dx.doi.org/10.1016/S0196-8858(02)00553-5">The site-perimeter of bargraphs</a>, Adv. in Appl. Math. 31 (2003), 86-112.
%H A274217 Wikipedia, <a href="https://en.wikipedia.org/wiki/Polyomino">Polyomino</a>
%F A274217 a(n) = Sum_{k>=4} k * A274207(k,n).
%Y A274217 Cf. A075126, A274207.
%K A274217 nonn
%O A274217 0,2
%A A274217 _Alois P. Heinz_, Jun 14 2016