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%I #18 Apr 04 2020 17:43:03
%S 1,1,3,9,43,264,2804,44524,1012456,30502320,1166185222,54902972542,
%T 3115560948081,209575881782122,16484822517084705,1498893370236782629,
%U 155996155818460127750,18424063865105027559797,2450806855748517847761175,364738110721163795332103692,60368334488355648041244493885
%N Number of non-isomorphic n X n nonnegative integer matrices with all row and column sums equal to 4 up to permutations of rows and columns.
%C Arises from counting of symmetric tensor invariants without color. See Geloun-Ramgoolam, Section 6.2 for information and Mathematica code.
%H Erick Castro, Itzhak Roditi, <a href="https://arxiv.org/abs/1804.08031">A combinatorial matrix approach for the generation of vacuum Feynman graphs multiplicities in phi-4 theory</a>, arXiv:1804.08031 [math-ph], 2018.
%H J. B. Geloun, S. Ramgoolam, <a href="http://arxiv.org/abs/1307.6490">Counting Tensor Model Observables and Branched Covers of the 2-Sphere</a>, arXiv preprint arXiv:1307.6490 [hep-th], 2013.
%Y Column k=4 of A333733.
%K nonn
%O 0,3
%A _N. J. A. Sloane_, Nov 22 2013
%E Name changed, a(0)=1 prepended and a(9)-a(20) from _Andrew Howroyd_, Apr 04 2020
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