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%I #4 Apr 08 2020 21:14:32
%S 1,1,3,9,33,142,742,4454,30516,233574,1972430,18190806,181774873,
%T 1955268162,22516241557,276294103003,3598106433662,49551992313623,
%U 719398127720459,10979514511427858,175716291405379723,2942195925987952412,51436169813668249174,937106275320950472387,17761791825490936694507
%N Number of non-isomorphic n X n nonnegative integer symmetric matrices with all row and column sums equal to 4 up to permutations of rows and columns.
%e The a(3) = 9 matrices are:
%e [0 0 4] [0 1 3] [0 1 3] [0 2 2] [0 2 2]
%e [0 4 0] [1 2 1] [1 3 0] [2 0 2] [2 1 1]
%e [4 0 0] [3 1 0] [3 0 1] [2 2 0] [2 1 1]
%e .
%e [1 0 3] [1 1 2] [1 1 2] [2 0 2]
%e [0 4 0] [1 2 1] [1 3 0] [0 4 0]
%e [3 0 1] [2 1 1] [2 0 2] [2 0 2]
%Y Column k=4 of A333737.
%K nonn
%O 0,3
%A _Andrew Howroyd_, Apr 08 2020
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