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 A209979 #14 May 05 2020 17:09:33
%S A209979 0,0,4,16,28,56,68,112,140,184,212,288,316,408,452,512,572,696,740,
%T A209979 880,940,1032,1108,1280,1340,1496,1588,1728,1820,2040,2100,2336,2460,
%U A209979 2616,2740,2928,3020,3304,3444,3632,3756,4072,4164,4496,4652,4840
%N A209979 Number of unimodular 2 X 2 matrices having all elements in {1,2,...,n}.
%C A209979 Equivalently, the number of 2 X 2 matrices having all elements in {1,2,...,n} and having an inverse whose elements are all integers.
%C A209979 See A210000 for a guide to related sequences.
%H A209979 Andrew Howroyd, <a href="/A209979/b209979.txt">Table of n, a(n) for n = 0..1000</a>
%F A209979 a(n) = 2*A196227(n) = 4*A209978(n). - _Andrew Howroyd_, May 05 2020
%t A209979 (See the Mathematica section at A210000.)
%Y A209979 Cf. A196227, A209978, A210000.
%K A209979 nonn
%O A209979 0,3
%A A209979 _Clark Kimberling_, Mar 16 2012