OFFSET
1,4
REFERENCES
R. Lie, Permutations and Patterns, Master's Thesis, Goeteborg, Sweden: Chalmers University of Technology, 2004.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..500
R. Parviainen, Lattice Path Enumeration of Permutations with k Occurrences of the Pattern 2-13, Journal of Integer Sequences, Vol. 9 (2006), Article 06.3.2.
FORMULA
a(n) = n*binomial(2*n,n-4)/2.
MATHEMATICA
Table[n Binomial[2 n, n - 4]/2, {n, 30}] (* Vincenzo Librandi, Aug 20 2015 *)
PROG
(PARI) a(n)=n*binomial(2*n, n-4)/2
(Magma) [n*Binomial(2*n, n-4)/2: n in [1..30]]; // Vincenzo Librandi, Aug 20 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Benoit Cloitre, May 27 2004
STATUS
approved