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 A209975 #10 Nov 22 2016 02:37:57
%S A209975 0,0,1,21,29,45,65,89,105,153,169,209,249,297,321,401,433,497,545,617,
%T A209975 649,769,809,897,977,1057,1105,1249,1297,1409,1489,1609,1673,1873,
%U A209975 1937,2033,2129,2273,2345,2585,2649,2809,2929,3097,3177,3369,3457
%N A209975 Number of 2 X 2 matrices having all elements in {0,1,...,n} and determinant 3.
%C A209975 See A210000 for a guide to related sequences.
%H A209975 Chai Wah Wu, <a href="/A209975/b209975.txt">Table of n, a(n) for n = 0..10000</a>
%t A209975 (See the Mathematica section at A210000.)
%Y A209975 Cf. A210000.
%K A209975 nonn
%O A209975 0,4
%A A209975 _Clark Kimberling_, Mar 16 2012