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A246840 G.f.: Sum_{n>=0} x^n * Sum_{k=0..n} C(n,k)^2 * x^(2*k). 6
1, 1, 1, 2, 5, 10, 18, 35, 73, 151, 306, 623, 1286, 2668, 5531, 11477, 23889, 49852, 104175, 217936, 456534, 957609, 2010839, 4226417, 8891022, 18719637, 39443860, 83170162, 175484915, 370491775, 782648333, 1654197568, 3498049053, 7400639286, 15664103420, 33168342557, 70260909811 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

Compare to the g.f. of Narayana's cows sequence A000930:

Sum_{n>=0} x^n * Sum_{k=0..n} C(n,k) * x^(2*k)  =  1/(1-x-x^3).

Compare to the g.f. of Whitney numbers sequence A051286:

Sum_{n>=0} x^n * Sum_{k=0..n} C(n,k)^2 * x^k = 1/sqrt((1+x+x^2)*(1-3*x+x^2)).

...

Limit a(n)/a(n+1) = t^2 = 0.465571231876768... where t = ((sqrt(93)+9)/18)^(1/3) - ((sqrt(93)-9)/18)^(1/3) is the positive real root of 1 - x - x^3 = 0.

LINKS

G. C. Greubel, Table of n, a(n) for n = 0..1000

FORMULA

G.f.: Sum_{n>=0} (2*n)!/(n!)^2 * x^(3*n) / (1 - x + x^3)^(2*n+1). - Paul D. Hanna, Oct 15 2014

G.f.: Sum_{n>=0} x^n * [Sum_{k>=0} C(n+k,k)^2 * x^(2*k)] * (1-x^2)^(2*n+1).

G.f.: Sum_{n>=0} x^(3*n) * [Sum_{k>=0} C(n+k,k)^2 * x^k].

G.f.: Sum_{n>=0} x^(3*n) * [Sum_{k=0..n} C(n,k)^2 * x^k] /(1-x)^(2n+1).

G.f.: exp( Sum_{n>=1} (x^n/n) * Sum_{k=0..n} C(2*n,2*k) * x^(2*k) ).

G.f.: exp( Sum_{n>=1} (x^n/n) * ((1+x)^(2*n) + (1-x)^(2*n))/2 ).

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

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

a(n) = Sum_{k=0..[n/2]} C(n-2*k, k)^2.

EXAMPLE

G.f.: A(x) = 1 + x + x^2 + 2*x^3 + 5*x^4 + 10*x^5 + 18*x^6 + 35*x^7 +...

where, by definition,

A(x) = 1 + x*(1 + x^2) + x^2*(1 + 2^2*x^2 + x^4)

+ x^3*(1 + 3^2*x^2 + 3^2*x^4 + x^6)

+ x^4*(1 + 4^2*x^2 + 6^2*x^4 + 4^2*x^6 + x^8)

+ x^5*(1 + 5^2*x^2 + 10^2*x^4 + 10^2*x^6 + 5^2*x^8 + x^10) +...

which is also given by the series identity:

A(x) = 1/(1-x+x^3) + 2*x^3/(1-x+x^3)^3 + 6*x^6/(1-x+x^3)^5 + 20*x^9/(1-x+x^3)^7 + 70*x^12/(1-x+x^3)^9 + 252*x^15/(1-x+x^3)^11 + 924*x^18/(1-x+x^3)^13 +...

The logarithm of the g.f. begins:

log(A(x)) = x*(1 + x^2) + x^2*(1 + 6*x^2 + x^4)/2

+ x^3*(1 + 15*x^2 + 15*x^4 + x^6)/3

+ x^4*(1 + 28*x^2 + 70*x^4 + 28*x^6 + x^8)/4

+ x^5*(1 + 45*x^2 + 210*x^4 + 210*x^6 + 45*x^8 + x^10)/5 +...

more explicitly,

log(A(x)) = x + x^2/2 + 4*x^3/3 + 13*x^4/4 + 26*x^5/5 + 46*x^6/6 + 99*x^7/7 + 229*x^8/8 + 499*x^9/9 + 1046*x^10/10 + 2223*x^11/11 + 4810*x^12/12 +...

where the logarithmic derivative equals

A'(x)/A(x) = (1-x+3*x^2+4*x^3-3*x^5)/((1-x+2*x^2-x^3)*(1-x-2*x^2-x^3)).

MATHEMATICA

CoefficientList[Series[1/Sqrt[(1 - x - x^3)^2 - 4*x^4], {x, 0, 50}], x] (* G. C. Greubel, Apr 27 2017 *)

PROG

(PARI) /* By definition: */

{a(n)=local(A=1); A=sum(m=0, n, x^m*sum(k=0, m, binomial(m, k)^2*x^(2*k)) +x*O(x^n)); polcoeff(A, n)}

for(n=0, 40, print1(a(n), ", "))

(PARI) /* From closed formula: */

{a(n)=local(A=1); A= 1/sqrt((1 - x - x^3)^2 - 4*x^4 +x*O(x^n)); polcoeff(A, n)}

for(n=0, 40, print1(a(n), ", "))

(PARI) /* From a series identity: */

{a(n)=local(A=1+x); A=sum(m=0, n, (2*m)!/(m!)^2 * x^(3*m) / (1 - x + x^3 +x*O(x^n))^(2*m+1)); polcoeff(A, n)}

for(n=0, 40, print1(a(n), ", "))

(PARI) /* From a binomial series identity: */

{a(n)=local(A=1+x); A=sum(m=0, n, x^m*(1-x^2)^(2*m+1)*sum(k=0, n, binomial(m+k, k)^2*x^(2*k)) +x*O(x^n)); polcoeff(A, n)}

for(n=0, 40, print1(a(n), ", "))

(PARI) /* From a binomial series identity: */

{a(n)=local(A=1+x); A=sum(m=0, n\3, x^(3*m)*sum(k=0, n-3*m, binomial(m+k, k)^2*x^k) +x*O(x^n)); polcoeff(A, n)}

for(n=0, 40, print1(a(n), ", "))

(PARI) /* From a binomial series identity: */

{a(n)=local(A=1+x); A=sum(m=0, n\3, x^(3*m) * sum(k=0, m, binomial(m, k)^2*x^k) / (1-x +x*O(x^n))^(2*m+1) ); polcoeff(A, n)}

for(n=0, 40, print1(a(n), ", "))

(PARI) /* From exponential formula: */

{a(n)=local(A=1); A=exp(sum(m=1, n, sum(k=0, m, binomial(2*m, 2*k)*x^(2*k)) * x^m/m) +x*O(x^n)); polcoeff(A, n)}

for(n=0, 40, print1(a(n), ", "))

(PARI) /* From exponential formula: */

{a(n)=local(A=1); A=exp(sum(m=1, n, ((1+x)^(2*m) + (1-x)^(2*m))/2 * x^m/m) +x*O(x^n)); polcoeff(A, n)}

for(n=0, 40, print1(a(n), ", "))

(PARI) /* From formula for a(n): */

{a(n)=sum(k=0, n\2, binomial(n-2*k, k)^2)}

for(n=0, 40, print1(a(n), ", "))

CROSSREFS

Cf. A246861, A181665, A246883, A246884.

Sequence in context: A281683 A224364 A327064 * A301885 A127297 A018739

Adjacent sequences:  A246837 A246838 A246839 * A246841 A246842 A246843

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Sep 04 2014

STATUS

approved

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Last modified July 10 11:42 EDT 2020. Contains 335576 sequences. (Running on oeis4.)