OFFSET
0,2
COMMENTS
a(n) equals the number of n-length words on {0,1,2,3} such that 0 appears only in a run whose length is a multiple of 4. - Milan Janjic, Feb 17 2015
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 900
Milan Janjic, Binomial Coefficients and Enumeration of Restricted Words, Journal of Integer Sequences, 2016, Vol 19, #16.7.3
Index entries for linear recurrences with constant coefficients, signature (3,0,0,1).
FORMULA
G.f.: 1/(1 - 3*x - x^4).
a(n) = 3*a(n-1) + a(n-4), with a(0)=1, a(1)=3, a(2)=9, a(3)=27.
a(n) = Sum_{alpha=RootOf(-1 + 3*z + z^4)} (1/2443)*(729 + 64*alpha + 144*alpha^2 + 324*alpha^3)*alpha^(-1-n).
MAPLE
spec := [S, {S=Sequence(Union(Z, Z, Z, Prod(Z, Z, Z, Z)))}, unlabeled]: seq(combstruct[count](spec, size=n), n=0..20);
seq(coeff(series(x^4/((1+2*x)*(2*x^3+x^2-2*x+1)), x, n+1), x, n), n = 0..30); # G. C. Greubel, Oct 16 2019
MATHEMATICA
CoefficientList[Series[1/(1-3x-x^4), {x, 0, 30}], x] (* Vincenzo Librandi, Feb 20 2015 *)
RecurrenceTable[{a[0]==1, a[1]==3, a[2]==9, a[3]==27, a[n]==3a[n-1] +a[n -4]}, a[n], {n, 0, 30}] (* Bruno Berselli, Feb 20 2015 *)
PROG
(PARI) Vec(1/(1-3*x-x^4) + O(x^30)) \\ Michel Marcus, Feb 17 2015
(Magma) [n le 4 select 3^(n-1) else 3*Self(n-1)+Self(n-4): n in [1..30]]; // Vincenzo Librandi, Feb 20 2015
(Sage)
def A052917_list(prec):
P.<x> = PowerSeriesRing(ZZ, prec)
return P(1/(1-3*x-x^4)).list()
A052917_list(30) # G. C. Greubel, Oct 16 2019
(GAP) a:=[1, 3, 9, 27];; for n in [5..30] do a[n]:=3*a[n-1]+a[n-4]; od; a; # G. C. Greubel, Oct 16 2019
(Magma) R<x>:=PowerSeriesRing(Integers(), 27); Coefficients(R!( 1/(1-3*x-x^4) )); // Marius A. Burtea, Oct 16 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
EXTENSIONS
More terms from James A. Sellers, Jun 06 2000
STATUS
approved