OFFSET
1,2
COMMENTS
a(n) is the number of compositions of n+9 into n parts avoiding parts 2 and 3. - Milan Janjic, Jan 07 2016
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
David Anderson, E. S. Egge, M. Riehl, L. Ryan, R. Steinke, Y. Vaughan, Pattern Avoiding Linear Extensions of Rectangular Posets, arXiv:1605.06825 [math.CO], 2016.
Colin Defant, Proofs of Conjectures about Pattern-Avoiding Linear Extensions, arXiv:1905.02309 [math.CO], 2019.
Index entries for linear recurrences with constant coefficients, signature (4,-6,4,-1).
FORMULA
Binomial transform of [1, 5, 5, 1, 0, 0, 0, ...].
G.f.: x*(1+2*x-2*x^2) / (1-x)^4. - R. J. Mathar, Apr 04 2012
a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4). - Vincenzo Librandi, Jun 29 2012
EXAMPLE
a(4) = 32 = sum of row 4, triangle A134464: (4 + 6 + 9 + 13).
a(4) = 32 = (1, 3, 3, 1) dot (1, 5, 5, 1) = (1 + 15 + 15 + 1).
MATHEMATICA
CoefficientList[Series[(1+2*x-2*x^2)/(1-x)^4, {x, 0, 50}], x] (* Vincenzo Librandi, Jun 29 2012 *)
PROG
(Magma) I:=[1, 6, 16, 32]; [n le 4 select I[n] else 4*Self(n-1)-6*Self(n-2)+4*Self(n-3)-Self(n-4): n in [1..50]]; // Vincenzo Librandi, Jun 29 2012
(PARI) Vec(x*(1+2*x-2*x^2)/(1-x)^4 + O(x^50)) \\ Altug Alkan, Jan 07 2016
(Sage) ((1+2*x-2*x^2)/(1-x)^4).series(x, 50).coefficients(x, sparse=False) # G. C. Greubel, May 08 2019
(GAP) a:=[1, 6, 16, 32];; for n in [5..50] do a[n]:=4*a[n-1]-6*a[n-2]+ 4*a[n-3]-a[n-4]; od; a; # G. C. Greubel, May 08 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Gary W. Adamson, Oct 26 2007
STATUS
approved