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 A284990 #15 Apr 10 2017 12:38:11
%S A284990 1,0,0,0,0,0,0,0,0,1,1,0,6,8,9,44,210,420,610,540,216,7570,33120,
%T A284990 66870,82080,66870,33120,7570,1975560,8171730,15729000,18433415,
%U A284990 14372820,7499940,2398900,357435,749649145,2971510080,5508175260,6267658544,4815171270,2570369760,932429820,209185200,22040361
%N A284990 Triangle T(n,t) read by rows: the number of n X n {0,1} matrices with trace t where each row sum and each column sum is 3.
%H A284990 Alois P. Heinz, <a href="/A284990/b284990.txt">Rows n = 0..11, flattened</a>
%H A284990 <a href="/index/Mat#binmat">Index entries for sequences related to binary matrices</a>
%e A284990 0: 1
%e A284990 1: 0 0
%e A284990 2: 0 0 0
%e A284990 3: 0 0 0 1
%e A284990 4: 1 0 6 8 9
%e A284990 5: 44 210 420 610 540 216
%e A284990 6: 7570 33120 66870 82080 66870 33120 7570
%e A284990 7: 1975560 8171730 15729000 18433415 14372820 7499940 2398900 357435
%Y A284990 Cf. A007107 (diagonal?), A001501 (row sums), A007105 (column 0?), A110040 (symmetric matrices).
%K A284990 nonn,tabl
%O A284990 0,13
%A A284990 _R. J. Mathar_, Apr 07 2017
%E A284990 More terms from _Alois P. Heinz_, Apr 09 2017