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 A226918 #14 Oct 02 2019 21:06:40 %S A226918 0,0,3,4,5,7,9,13,16,20,24,28 %N A226918 Minimal number of 1X3 I-trominoes needed to prevent any further I-trominoe from being placed on an n X n grid. %C A226918 The I-trominoes must line up with the squares on the grid and must occupy exactly three squares (skew placements are not allowed). %C A226918 a(n) is the independent domination number of a graph with one node per tromino and an edge for each pair of trominoes that conflict. - _Rob Pratt_, Oct 02 2019 %H A226918 J. Cernenoks and A. Cibulis, <a href="http://aict.itf.llu.lv/files/rakstkraj/2013/Cernenoks_AICT2013.pdf">Application of IT in Mathematical Proofs and in Checking of Results of Pupils' Research</a>, International Conference on Applied Information and Communication Technologies (AICT2013), 25.-26. April, 2013, Jelgava, Latvia. %H A226918 J. Cernenoks and A. Cibulis, <a href="/A226918/a226918.jpg">Illustration of initial terms</a> (Figure 11 from the above reference, included with permission of the authors) %K A226918 nonn,more %O A226918 1,3 %A A226918 _N. J. A. Sloane_, Jul 11 2013 %E A226918 a(12) from _Rob Pratt_, Oct 02 2019