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 A065128 #33 Jul 06 2025 02:54:24
%S A065128 1,2,96,86016,1321205760,335522845163520,1385295986380096143360,
%T A065128 92239345887620476544860815360,98654363640526679389774053813465907200,
%U A065128 1691558770638735027870457216848672340872423014400,464518059995994038184379206447729320401459864818351813427200
%N A065128 Number of invertible n X n matrices mod 4 (i.e., over the ring Z_4).
%H A065128 Geoffrey Critzer, <a href="https://esirc.emporia.edu/handle/123456789/3595">Combinatorics of Vector Spaces over Finite Fields</a>, Master's thesis, Emporia State University, 2018.
%H A065128 Jeffrey Overbey, William Traves, and Jerzy Wojdylo, <a href="https://doi.org/10.1080/0161-110591893771">On the Keyspace of the Hill Cipher</a>, Cryptologia, Vol. 29, Iss. 1 (2005), pp. 59-72; <a href="http://jeff.over.bz/papers/undergrad/on-the-keyspace-of-the-hill-cipher.pdf">author's copy</a>.
%F A065128 a(n) = 4^(n^2) * Product_{k=1..n} (1 - 1/2^k).
%F A065128 a(n) = 2^(n^2) * A002884(n). - _Geoffrey Critzer_, Feb 04 2018
%F A065128 From _Amiram Eldar_, Jul 06 2025: (Start)
%F A065128 a(n) = Product_{k=1..n} 2*A060195(k).
%F A065128 a(n) ~ c * 4^(n^2), where c = A048651. (End)
%t A065128 a[n_] := 4^(n^2)*Product[1 - 1/2^k, {k, 1, n} ]; Table[ a[n], {n, 0, 10} ]
%o A065128 (PARI) for(n=1,11,print(4^(n^2)*prod(k=1,n,(1-1/2^k))))
%Y A065128 Column k=4 of A316622.
%Y A065128 Cf. A048651, A053291, A060195.
%K A065128 nonn,easy
%O A065128 0,2
%A A065128 Dan Fux (dan.fux(AT)OpenGaia.com or danfux(AT)OpenGaia.com), Nov 14 2001
%E A065128 More terms from _Robert G. Wilson v_ and _Jason Earls_, Nov 16 2001