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 A206258 #21 Dec 23 2024 14:53:43 %S A206258 5,13,29,45,77,93,141,173,221,253,333,365,461,509,573,637,765,813,957, %T A206258 1021,1117,1197,1373,1437,1597,1693,1837,1933,2157,2221,2461,2589, %U A206258 2749,2877,3069,3165,3453,3597,3789,3917,4237,4333,4669,4829,5021,5197,5565 %N A206258 1/8 the number of 2 X 2 -n..n arrays with a 2 X 2 -n..n inverse, i.e., with determinant +-1. %H A206258 R. H. Hardin, <a href="/A206258/b206258.txt">Table of n, a(n) for n = 1..334</a> %H A206258 R. H. Hardin and Max Alekseyev, <a href="https://web.archive.org/web/*/http://list.seqfan.eu/oldermail/seqfan/2012-February/008974.html">2X2 -n..n with inverse in -n..n and totient function?</a>, Seqfan list Feb 2012. %F A206258 a(n) = a(n-1) + 8*A000010(n). %F A206258 From _Amiram Eldar_, Dec 13 2024: (Start) %F A206258 a(n) = 8*A002088(n) - 3. %F A206258 a(n) = (24/Pi^2) * n^2 + O(n*log(n)). (End) %t A206258 8* Accumulate[EulerPhi[Range[50]]] - 3 (* _Amiram Eldar_, Dec 13 2024 *) %Y A206258 Cf. A000010, A002088. %K A206258 nonn %O A206258 1,1 %A A206258 _R. H. Hardin_ with _Max Alekseyev_, Feb 05 2012