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 A057981 #24 Feb 16 2025 08:32:43 %S A057981 0,1,33,7875,15099201,237634987683,30805715676309201 %N A057981 Number of singular n X n (-1,0,1)-matrices. %H A057981 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/SingularMatrix.html">Singular Matrix.</a> %H A057981 Minfeng Wang, <a href="/A057981/a057981.cpp.txt">C++ program to calculate A057981</a> %F A057981 a(n) = A060722(n) - A056989(n). - _Alois P. Heinz_, Dec 02 2019 %Y A057981 Complement of A056989. %Y A057981 Cf. A060722, A330134. %K A057981 nonn,more %O A057981 0,3 %A A057981 _Eric W. Weisstein_, Oct 23 2000 %E A057981 a(5) from _Giovanni Resta_, Feb 20 2009 %E A057981 a(0)=0 prepended by _Alois P. Heinz_, Dec 02 2019 %E A057981 a(0)-a(5) confirmed and a(6) added by _Minfeng Wang_, May 01 2024