OFFSET
0,2
LINKS
Lily Yen, Table of n, a(n) for n = 0..99
Wei Chen, Enumeration of Set Partitions Refined by Crossing and Nesting Numbers, MS Thesis, Department of Mathematics. Simon Fraser University, Fall 2014. Table 5.2, r=3.
Lily Yen, Crossings and Nestings for Arc-Coloured Permutations and Arc-coloured permutations, PSAC 2013, Paris, France, June 24-28, Proc. DMTCS (2013) 743-754.
Index entries for linear recurrences with constant coefficients, signature (20,-108,144).
FORMULA
G.f.: (1-17*x+66*x^2-36*x^3)/((1-2*x)*(1-6*x)*(1-12*x)).
a(n) = 9*2^n/20 +6^n/4 +12^n/20, n>0. - R. J. Mathar, Jun 11 2019
EXAMPLE
For n=3, a(3)= 144, the number of ways to color arcs of a permutation on {1,2,3} in 3 colors such that the arcs neither cross nor nest.
MATHEMATICA
Join[{1}, LinearRecurrence[{20, -108, 144}, {3, 18, 144}, 20]] (* Jean-François Alcover, Jul 22 2018 *)
PROG
(PARI) Vec((1-17*x+66*x^2-36*x^3)/((1-2*x)*(1-6*x)*(1-12*x))+O(x^66)) \\ Joerg Arndt, Apr 24 2013
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Lily Yen, Apr 24 2013
STATUS
approved