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Number of 2n X 2n symmetric matrices whose first row is 1..2n and whose rows and columns are all permutations of 1..2n.
3

%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