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 A333165 Number of non-isomorphic 2n X 2n symmetric matrices with entries in {+1, -1} and all rows and columns summing to zero. 3
 1, 1, 2, 5, 66, 7937, 10211144, 133506398361, 18551599312980440, 28652629505982770906471, 510824181488832447063505273252 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS a(n) is the number of non-isomorphic 2n X 2n symmetric binary matrices with n ones in every row and column. LINKS FORMULA a(n) = A333159(2*n, n). EXAMPLE The a(1) = 1 matrix is:   [+ -]   [- +] The a(2) = 2 matrices are:   [+ + - -]   [+ + - -]   [+ + - -]   [+ - + -]   [- - + +]   [- + - +]   [- - + +]   [- - + +] CROSSREFS Central coefficients of A333159. Cf. A333164, A333166. Sequence in context: A305292 A268211 A246280 * A133004 A321602 A175169 Adjacent sequences:  A333162 A333163 A333164 * A333166 A333167 A333168 KEYWORD nonn,more AUTHOR Andrew Howroyd, Mar 11 2020 STATUS approved

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Last modified September 27 19:34 EDT 2021. Contains 347694 sequences. (Running on oeis4.)