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 A277876 a(n) = n!/(m*(n-m)) with m = floor(n/2). 1
 2, 3, 6, 20, 80, 420, 2520, 18144, 145152, 1330560, 13305600, 148262400, 1779148800, 23351328000, 326918592000, 4940103168000, 79041650688000, 1351612226764800, 24329020081766400, 464463110651904000, 9289262213038080000, 195848611658219520000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,1 COMMENTS Consider this practical problem: n > 1 people are to be seated at two labeled round tables (T1 and T2), m of them at table T1, the rest at table T2. Two such seatings (A and B) are considered distinct if at least one person does not sit at the same table in seating A as in seating B, or has a different left or right neighbor (while rotating the seatings around any of the two tables is irrelevant). The number of such seatings is clearly binomial(n,m)*(m-1)!*(n-m-1)! which simplifies to this a(n). The formula holds for any m satisfying 0 < 2*m <= n. LINKS Stanislav Sykora, Table of n, a(n) for n = 2..201 MAPLE a:= n-> (m->n!/(m*(n-m)))(floor(n/2)): seq(a(n), n=2..30);  # Alois P. Heinz, Nov 04 2016 MATHEMATICA Table[n! / (Floor[n/2] (n - Floor[n/2])), {n, 2, 25}] (* Vincenzo Librandi, Nov 04 2016 *) PROG (MAGMA) [Factorial(n)/(Floor(n/2)*(n-Floor(n/2))): n in [2..30]]; // Vincenzo Librandi, Nov 04 2016 CROSSREFS Sequence in context: A227316 A176806 A168268 * A002078 A000372 A123930 Adjacent sequences:  A277873 A277874 A277875 * A277877 A277878 A277879 KEYWORD nonn AUTHOR Stanislav Sykora, Nov 03 2016 STATUS approved

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Last modified December 16 07:12 EST 2018. Contains 318158 sequences. (Running on oeis4.)