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A103770 A weighted tribonacci sequence, (1,3,9). 1
1, 1, 4, 16, 37, 121, 376, 1072, 3289, 9889, 29404, 88672, 265885, 796537, 2392240, 7174816, 21520369, 64574977, 193709428, 581117680, 1743420757, 5230158649, 15690480040, 47071742800, 141214610761, 423644159521, 1270933677004 (list; graph; refs; listen; history; text; internal format)



The weighted tribonacci (1,r,r^2) with g.f. 1/(1-x-r*x^2-r^2x^3) has general term sum{k=0..n, T(n-k,k)r^k}.

Correspondence: a(n)=b(n+2)*3^n, where b(n) is the sequence of the arithmetic means of the previous three terms defined by b(n)=1/3*(b(n-1)+b(n-2)+b(n-3)) with initial values b(0)=0, b(1)=0, b(2)=1; The g.f. for b(n) is B(x):=x^2/(1-(x^1+x^2+x^3)/3), so the g.f. A(x) for a(n) satisfies A(x)=B(3*x)/(3*x)^2. Because b(n) converges to the limit lim (1-x)*B(x)=1/6*(b(0)+2*b(1)+3*b(2))=1/2 (for x-->1), it follows that a(n)/3^n also converges to 1/2. This correspondence is valid in general (with necessary changes) for weighted sequences of order (1,p,p^2,p^3,p^4,...,p^(p-1)) with natural p>0. Forming such sequences c(n):=c(n-1)+p^1*c(n-2)+...+p^(p-1)*c(n-p) the limit of c(n)/p^n is 2/(p+1) (see also A001045). - Hieronymus Fischer, Feb 04 2006

a(n)/3^n equals the probability that n will occur as a partial sum in a randomly-generated infinite sequence of 1s, 2s and 3s.  The limit ratio is 1/2. - Bob Selcoe, Jul 05 2013

Number of compositions of n into one sort of 1's, three sorts of 2's, and nine sorts of 3's. [Joerg Arndt, Jul 06 2013]


Table of n, a(n) for n=0..26.

Index entries for linear recurrences with constant coefficients, signature (1,3,9).


G.f.: 1/(1-x-3*x^2-9*x^3).

a(n) = sum{k=0..n, T(n-k, k)3^k}, T(n, k) = trinomial coefficients (A027907).

a(n)=sum{k=0..n, sum{i=0..floor((n-k)/2), C(n-k-i, i)C(k, n-k-i)}*3^(n-k)}; - Paul Barry, Apr 26 2005

a(n)/3^n converges to 1/2. - Hieronymus Fischer, Feb 02 2006

a(0)=1, a(1)=1, a(2)=4, a(n)=a(n-1)+3*a(n-2)+9*a(n-3), n>=3; - Hieronymus Fischer, Feb 04 2006

a(n) = 3^n + b(n) + b(n-1), with b(n) = (-1)^A121262(n+1)*A088137(n+1). - Ralf Stephan, May 20 2007


Cf. A000073, A102001.

Cf. A071675.

Sequence in context: A080855 A203299 A198015 * A121318 A152133 A210440

Adjacent sequences:  A103767 A103768 A103769 * A103771 A103772 A103773




Paul Barry, Feb 15 2005



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Last modified February 12 21:30 EST 2016. Contains 268229 sequences.