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A333164
Number of 2n X 2n symmetric matrices with entries in {+1, -1} and all rows and columns summing to zero.
4
1, 2, 18, 1760, 1944530, 26615510712, 4762109992158288, 11528251571501588791296, 386860001875783390762182911250, 183238648400953515891207813311894423000, 1242327573587456839123512395835593196519781229768, 121917941188016997420391711911475819481799846311888884541952
OFFSET
0,2
COMMENTS
a(n) is the number of 2n X 2n symmetric binary matrices with n ones in every row and column.
FORMULA
a(n) = A333157(2*n, n).
a(n) = A213793(2*n). - Andrew Howroyd, Apr 08 2020
EXAMPLE
The a(1) = 2 matrices are:
[ 1 -1] [-1 1]
[-1 1] [ 1 -1]
CROSSREFS
Central coefficients of A333157.
Sequence in context: A155206 A334623 A260610 * A076954 A206847 A259654
KEYWORD
nonn
AUTHOR
Andrew Howroyd, Mar 11 2020
STATUS
approved