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 A139670 #27 Mar 03 2024 08:20:09
%S A139670 1,26,820,35150,1944530,133948836,11234051976,1127512146540,
%T A139670 133475706272700,18406586045919060,2925154024273348296,
%U A139670 530686776655470875076,109004840145995702773410,25164525076896596670014400,6486836210471246515195539840,1856264107759263993451053077856
%N A139670 Number of n X n symmetric binary matrices with all row sums 4.
%C A139670 From _R. J. Mathar_, Apr 07 2017: (Start)
%C A139670 These are the row sums of the following triangle, which shows in row n and column t the number of symmetric n X n {0,1}-matrices with trace t and 4 ones in each row and each column, 0 <= t <= n:
%C A139670 0: 1;
%C A139670 1: 0, 0;
%C A139670 2: 0, 0, 0;
%C A139670 3: 0, 0, 0, 0;
%C A139670 4: 0, 0, 0, 0, 1;
%C A139670 5: 1, 0, 10, 0, 15, 0;
%C A139670 6: 15, 0, 270, 0, 465, 0, 70;
%C A139670 7: 465, 0, 9660, 0, 19355, 0, 5670, 0;
%C A139670 (End)
%H A139670 Andrew Howroyd, <a href="/A139670/b139670.txt">Table of n, a(n) for n = 4..30</a>
%e A139670 a(4) = 1:
%e A139670 1 1 1 1
%e A139670 1 1 1 1
%e A139670 1 1 1 1
%e A139670 1 1 1 1
%Y A139670 Column k=4 of A333157 and row 4 of A188448.
%Y A139670 Cf. A000085 (row sums 1), A000986 (row sums 2), A110040 (row sums 3).
%K A139670 nonn
%O A139670 4,2
%A A139670 _R. H. Hardin_, Jun 12 2008
%E A139670 Terms a(13) and beyond from _Andrew Howroyd_, Mar 09 2020