login
This site is supported by donations to The OEIS Foundation.

 

Logo

Annual Appeal: Please make a donation to keep the OEIS running. In 2018 we replaced the server with a faster one, added 20000 new sequences, and reached 7000 citations (often saying "discovered thanks to the OEIS").
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

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