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%I #11 May 17 2020 01:42:06
%S 1,1,3,15,199,6247,505623,105997283,58923059879,87882127554983,
%T 355824020816195749,3945677945542866439849,
%U 120747614641791880577007593,10263027079252827219477164325361,2436143827877272419846561427146618539,1622665432590314350761059585434700996826279
%N Number of lower triangles of an n X n -1..1 array with all row and column sums zero.
%e Some solutions for 4 X 4:
%e ..0........0........0........0........0........0........0........0
%e ..1-1.....-1.1......1-1......0.0.....-1.1......0.0.....-1.1......1-1
%e ..0.0.0....1-1.0....0.0.0....0.0.0....0.0.0....1.0-1....0.0.0...-1.1.0
%e .-1.1.0.0..0.0.0.0.-1.1.0.0..0.0.0.0..1-1.0.0.-1.0.1.0..1-1.0.0..0.0.0.0
%Y Column 1 of A195522 and column 2 of A195644.
%K nonn
%O 1,3
%A _R. H. Hardin_, Sep 20 2011
%E a(13)-a(16) from _Andrew Howroyd_, May 16 2020
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