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A014531 Form array in which n-th row is obtained by expanding (1+x+x^2)^n and taking the 2nd column from the center. 12
1, 3, 10, 30, 90, 266, 784, 2304, 6765, 19855, 58278, 171106, 502593, 1477035, 4343160, 12778152, 37616427, 110797569, 326527350, 962803170, 2840372304, 8383467708, 24755608584, 73133433800, 216143407675, 639062383401 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Number of "up" steps in all Motzkin paths of length n+1. E.g. a(2)=3 because in the four Motzkin paths of length 3, HHH, HUD, UDH and UHD, where H=(1,0), U=(1,1), D=(1,-1), we have altogether three U steps. - Emeric Deutsch, Dec 26 2003

a(n-1) = A111808(n,n-2) for n>1. - Reinhard Zumkeller, Aug 17 2005

a(n) = number of paths in the half-plane x>=0, from (0,0) to (n+1,2), and consisting of steps U=(1,1), D=(1,-1) and H=(1,0). For example, for n=2, we have the 3 paths: UUH, HUU, UHU. - José Luis Ramírez Ramírez, Apr 19 2015

REFERENCES

L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 78.

LINKS

G. C. Greubel, Table of n, a(n) for n = 1..1000 (terms 1..200 from T. D. Noe)

Eric Weisstein's World of Mathematics, Trinomial Coefficient.

FORMULA

a(n) = A002426(n+1)-A001006(n+1) = a(n-1)+A005717(n)+A014532(n-2) - Henry Bottomley, May 15 2001

E.g.f.: exp(x)*(2*x*BesselI(1, 2*x)+(x-2)*BesselI(2, 2*x))/x. - Vladeta Jovovic, Aug 21 2003

G.f.: [1-2z-z^2-(1-z)q]/(2z^3q), where q=sqrt(1-2z-3z^2). - Emeric Deutsch, Dec 26 2003

a(n) = sum{k=0..n+1, C(n+1,k)*C(n-k+1,k+2)} - Paul Barry, Sep 20 2004

Apparently (n+3)*(n-1)*a(n) -(n+1)*(2n+1)*a(n-2)-3*n*(n+1)*a(n-2)=0. - R. J. Mathar, Dec 08 2011

a(n) = n*(n+1)*hypergeom([(1-n)/2, 1-n/2], [3], 4)/2. - Peter Luschny, Nov 23 2014

G.f.: z*M(z)^2/(1-z-2*z^2*M(z)), where M(z) is the g.f. of Motzkin paths. - José Luis Ramírez Ramírez, Apr 19 2015

a(n) = GegenbauerC(n-1, -n-1, -1/2). - Peter Luschny, May 09 2016

MAPLE

seq( add(binomial(i+1, k)*binomial(i-k+1, k+2), k=0..floor(i/2)), i=1..30 ); # Detlef Pauly (dettodet(AT)yahoo.de), Nov 09 2001

a := n -> simplify(GegenbauerC(n-1, -n-1, -1/2)):

seq(a(n), n=1..26); # Peter Luschny, May 09 2016

MATHEMATICA

Table[Sum[Binomial[i + 1, k]*Binomial[i - k + 1, k + 2], {k, 0, Floor[i/2]}], {i, 30}] (* Michael De Vlieger, Apr 20 2015 *)

Table[GegenbauerC[n - 1, -n - 1, -1/2], {n, 1, 50}] (* G. C. Greubel, Feb 28 2017 *)

PROG

(Sage)

a = lambda n: n*(n+1)*hypergeometric([(1-n)/2, 1-n/2], [3], 4)/2

[simplify(a(n)) for n in (1..26)] # Peter Luschny, Nov 23 2014

(PARI) for(n=1, 25, print1(sum(k=0, n+1, binomial(n+1, k)*binomial(n-k+1, k+2)), ", ")) \\ G. C. Greubel, Feb 28 2017

CROSSREFS

Cf. A027907, A005717.

First differences are in A025180.

Sequence in context: A261336 A026109 A026327 * A062107 A269800 A033113

Adjacent sequences:  A014528 A014529 A014530 * A014532 A014533 A014534

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane

EXTENSIONS

More terms from James A. Sellers, Feb 05 2000

STATUS

approved

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Last modified November 17 08:16 EST 2018. Contains 317275 sequences. (Running on oeis4.)