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 A306820 #6 Mar 12 2019 04:51:07
%S A306820 1,9,481,148817,243782721
%N A306820 Number of nilpotent n X n matrices with elements {-1, 0, 1}.
%H A306820 S. E. Thornton, <a href="http://www.bohemianmatrices.com/cpdb/unstructured/unstructured_n1_0_1">Properties of the Bohemian family of n x n matrices with population {-1, 0, 1}</a>, Characteristic Polynomial Database.
%Y A306820 Number of characteristic polynomials is in A272658.
%Y A306820 Number of distinct eigenvalues is in A271570.
%K A306820 nonn,more,hard
%O A306820 1,2
%A A306820 _Steven E. Thornton_, Mar 11 2019