OFFSET
1,2
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..235
Ignacio Larrosa Cañestro, Marko Riedel, n-digonos.
Marko Riedel, Pairs of beads on a ring.
FORMULA
a(n) = ((2n)!/2^n + (2n+1)*n!)/(4n).
a(n) ~ sqrt(Pi)*2^n*n^(2*n-1/2)/(2*exp(2*n)). - Ilya Gutkovskiy, Nov 21 2016
EXAMPLE
a(2) = 2 because there are two quadrilaterals with sides {1, 1, 2, 2}: a kite and a rectangle.
MATHEMATICA
Table[((2 n)! / 2^n + (n + 1) n! + n n!) / (4 n), {n, 20}] (* Vincenzo Librandi, Nov 22 2016 *)
PROG
(Magma) [(Factorial(2*n)/2^n+(n+1)*Factorial(n)+n*Factorial(n))/(4*n): n in [1..20]]; // Vincenzo Librandi, Nov 22 2016
(PARI) for(n=1, 25, print1(((2*n)!/2^n + (2*n+1)*n!)/(4*n), ", ")) \\ G. C. Greubel, May 25 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Ignacio Larrosa Cañestro, Jul 19 2006
EXTENSIONS
Name clarified by Marko Riedel, Nov 22 2016
STATUS
approved