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 A194729 Number of 9-ary 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. 5
 1, 1, 17, 353, 8113, 198401, 5060433, 133071009, 3581326065, 98156060225, 2730108129937, 76862217117665, 2186096427128369, 62718004238927233, 1812849590253944273, 52742324721313632033, 1543272031837984426353, 45386639860532255882433, 1340844916965007902013713 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Alois P. Heinz, Table of n, a(n) for n = 0..200 FORMULA G.f.: 8/9 + 16/(9*(7+9*sqrt(1-32*x))). a(0) = 1, a(n) = 1/n * Sum_{j=0..n-1} C(2*n,j)*(n-j)*8^j for n>0. a(n) ~ 8 * 32^n / (49 * sqrt(Pi) * n^(3/2)). - Vaclav Kotesovec, Sep 07 2014 n*a(n) +(-113*n+48)*a(n-1) +1296*(2*n-3)*a(n-2)=0. - R. J. Mathar, Mar 14 2015 From Karol A. Penson, Jul 15 2015: (Start) Special values of the hypergeometric function 2F1, in Maple notation: a(n+1) = 2^8*32^n*GAMMA(n+3/2)*hypergeom([1,n+3/2],[n+3],32/81)/(81*sqrt(Pi)*(n+2)!), n=0,1,... . Integral representation as the n-th moment of a positive function W(x) = sqrt(y*(32-y))/(2*Pi*(81-y)),y=0..32) on (0,32): a(n+1) = int(x^n*W(x), x=0..32), n=0,1,... . This representation is unique as W(x) is the solution of the Hausdorff moment problem. (End) EXAMPLE a(2) = 17: aaaa, aabb, aacc, aadd, aaee, aaff, aagg, aahh, aaii, abba, acca, adda, aeea, affa, agga, ahha, aiia (with 9-ary alphabet {a,b,c,d,e,f,g,h,i}). MAPLE a:= n-> `if`(n=0, 1, add(binomial(2*n, j) *(n-j) *8^j, j=0..n-1) /n): seq(a(n), n=0..20); MATHEMATICA CoefficientList[Series[8/9 + 16/(9 (7 + 9 Sqrt[1 - 32 x])), {x, 0, 33}], x] (* Vincenzo Librandi, Jul 16 2015 *) CROSSREFS Column k=9 of A183134. Cf. A194723, A194726, A174728. Sequence in context: A171860 A324449 A191589 * A081421 A121824 A120287 Adjacent sequences:  A194726 A194727 A194728 * A194730 A194731 A194732 KEYWORD nonn AUTHOR Alois P. Heinz, Sep 02 2011 STATUS approved

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Last modified September 24 07:28 EDT 2020. Contains 337317 sequences. (Running on oeis4.)