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 A219975 #25 Apr 09 2025 22:57:18 %S A219975 1,0,1,2,3,28,267,2744,66653,2766100,141365332,13305552648, %T A219975 2149055591278,493880634209398,192321197859269019, %U A219975 124351154502319720238,122893248485909264026734,199405053536180281080458422,527809383857797224536981601752 %N A219975 Number of tilings of an n X n square using straight (3 X 1) trominoes and 2 X 2 tiles. %H A219975 Kai Liang, <a href="https://arxiv.org/abs/2503.17698">Solving tiling enumeration problems by tensor network contractions</a>, arXiv:2503.17698 [math.CO], 2025. See p. 25, Table 4. %e A219975 a(4) = 3, because there are 3 tilings of a 4 X 4 square using straight (3 X 1) trominoes and 2 X 2 tiles: %e A219975 ._._____. ._____._. ._._._._. %e A219975 | |_____| |_____| | | . | . | %e A219975 | | . | | | | . | | |___|___| %e A219975 |_|___| | | |___|_| | . | . | %e A219975 |_____|_| |_|_____| |___|___| . %Y A219975 Main diagonal of A219967. %Y A219975 Cf. A233807. %K A219975 nonn,more %O A219975 0,4 %A A219975 _Alois P. Heinz_, Dec 02 2012 %E A219975 a(12) from _Alois P. Heinz_, Sep 24 2014 %E A219975 a(13)-a(18) from _Martin Fuller_, Apr 09 2025