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%I #11 Dec 23 2021 21:52:02
%S 7,121,5011,493189,116069407,65255935753,87660979427131,
%T 281363248472730805,2157777098414728716343,39538807262703057593016241
%N a(n) is 1/4 the number of (n+1) X (n+1) 0..3 symmetric matrices with every 2 X 2 subblock having one or three distinct values.
%e Some solutions for n=3:
%e 1 2 0 3 2 3 2 0 3 1 3 2 3 2 1 1 1 1 2 2
%e 2 0 1 1 3 0 3 2 1 2 3 0 2 1 0 3 1 1 3 0
%e 0 1 2 0 2 3 1 2 3 3 1 0 1 0 2 3 2 3 0 1
%e 3 1 0 1 0 2 2 0 2 0 0 2 1 3 3 1 2 0 1 2
%K nonn,more
%O 1,1
%A _R. H. Hardin_, Mar 31 2012
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