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%I #25 Jun 17 2022 03:18:24
%S 1,1,4,456,10936320,130025295912960,2209617218725251404267520
%N Number of 2n X 2n symmetric matrices whose first row is 1..2n and whose rows and columns are all permutations of 1..2n.
%F a(n) = A035481(2*n). - _Max Alekseyev_, Apr 23 2010
%Y Cf. A000438, A035481, A000315, A002860, A003090, A040082.
%K more,nonn,nice
%O 0,3
%A _Joshua Zucker_ and Joe Keane
%E a(5)-a(6) from _Ian Wanless_, Oct 20 2019
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