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A194724 Number of quaternary words either empty or beginning with the first character of the alphabet, that can be built by inserting n doublets into the initially empty word. 4
1, 1, 7, 58, 523, 4966, 48838, 492724, 5068915, 52955950, 560198962, 5987822380, 64563867454, 701383563388, 7668869344108, 84326618668648, 931894610845123, 10344218506421758, 115280448164645818, 1289346114476360188, 14467472108268263818, 162816535672067515828 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

REFERENCES

C. Kassel and C. Reutenauer, Algebraicity of the zeta function associated to a matrix over a free group algebra, arXiv preprint arXiv:1303.3481, 2013

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..500

FORMULA

G.f.: 3/4 + 3/(2*(2+4*sqrt(1-12*x))).

a(0) = 1, a(n) = 1/n * Sum_{j=0..n-1} C(2*n,j)*(n-j)*3^j for n>0.

EXAMPLE

a(2) = 7: aaaa, aabb, aacc, aadd, abba, acca, adda (with quaternary alphabet {a,b,c,d}).

MAPLE

a:= n-> `if` (n=0, 1, add(binomial(2*n, j) *(n-j) *3^j, j=0..n-1)/n):

seq (a(n), n=0..25);

# second Maple program:

a:= proc(n) option remember; `if`(n<3, [1, 1, 7][n+1],

      ((28*n-18)*a(n-1) -(192*n-288)*a(n-2))/n)

    end:

seq(a(n), n=0..30);

MATHEMATICA

CoefficientList[Series[3/4+3/(2(2+4Sqrt[1-12x])), {x, 0, 30}], x] (* Harvey P. Dale, Sep 30 2012 *)

CROSSREFS

Column k=4 of A183134.

Sequence in context: A238810 A006193 A139397 * A081343 A163048 A192940

Adjacent sequences:  A194721 A194722 A194723 * A194725 A194726 A194727

KEYWORD

nonn

AUTHOR

Alois P. Heinz, Sep 02 2011

STATUS

approved

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Last modified April 24 00:51 EDT 2014. Contains 240947 sequences.