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 A035481 Number of n X n symmetric matrices whose first row is 1..n and whose rows and columns are all permutations of 1..n. 10
 1, 1, 1, 1, 4, 6, 456, 6240, 10936320, 1225566720, 130025295912960, 252282619805368320, 2209617218725251404267520, 98758655816833727741338583040 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS The odd subsequence is A000438. The even subsequence is A035483. LINKS Brendan D. McKay and Ian M. Wanless, Enumeration of Latin squares with conjugate symmetry, J. Combin. Des. 30 (2022), 105-130, also on Enumeration of Latin squares with conjugate symmetry, arXiv:2104.07902 [math.CO], 2021. Table 2 p. 7. EXAMPLE a(3) = 1 because after 123 in the first row and column, 213 is not allowed for the second row, so it must be 231 and thus the third row is 312. MATHEMATICA (* This script is not suitable for n > 6 *) matrices[n_ /; n > 1] := Module[{a, t, vars}, t = Table[Which[i==1, j, j==1, i, j>i, a[i, j], True, a[j, i]], {i, n}, {j, n}]; vars = Select[Flatten[t], !IntegerQ[#]& ] // Union; t /. {Reduce[And @@ (1 <= # <= n & /@ vars) && And @@ Unequal @@@ t, vars, Integers] // ToRules}]; a[0] = a[1] = 1; a[n_] := Length[ matrices[n]]; Table[an = a[n]; Print["a(", n, ") = ", an]; an, {n, 0, 6}] (* Jean-François Alcover, Jan 04 2016 *) CROSSREFS Cf. A000438, A035482, A000315, A002860, A003090, A040082. Sequence in context: A113838 A056831 A027717 * A323214 A061214 A137024 Adjacent sequences:  A035478 A035479 A035480 * A035482 A035483 A035484 KEYWORD nonn,more,nice AUTHOR Joshua Zucker and Joe Keane EXTENSIONS a(10)-a(13) from Ian Wanless, Oct 20 2019 STATUS approved

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Last modified January 20 10:18 EST 2022. Contains 350471 sequences. (Running on oeis4.)