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A245926 G.f.: sqrt( (1-x + sqrt(1-14*x+x^2)) / (2*(1-14*x+x^2)) ). 8
1, 5, 51, 587, 7123, 89055, 1135005, 14660805, 191253843, 2513963567, 33244446601, 441772827105, 5894323986301, 78912561223553, 1059543126891027, 14261959492731387, 192392702881384275, 2600355510685245087, 35206018016510388345, 477377227987055971905 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Square each term to form a bisection of A245925.

Limit a(n+1)/a(n) = 7 + 4*sqrt(3).

LINKS

G. C. Greubel, Table of n, a(n) for n = 0..870

FORMULA

a(n)^2 = Sum_{k=0..2*n} Sum_{j=0..4*n-2*k} (-1)^(j+k) * C(4*n-k,j+k)^2 * C(j+k,k)^2.

a(n) ~ (3*sqrt(3)-5) * (7+4*sqrt(3))^(n+1) / (4*sqrt(Pi*n)). - Vaclav Kotesovec, Aug 16 2014

a(n)^2 = C(4*n,2*n)*hyper4F3([-2*n,-2*n,-2*n,-2*n+1/2],[1,-4*n,-4*n],4). - Peter Luschny, Aug 17 2014

a(n)^2 = Sum{k=0..2*n} binomial(4*n-2*k, 2*n-k)*binomial(4*n-k, k)^2). - Peter Luschny, Aug 17 2014

a(n)^2 = Sum_{k=0..2*n} (-1)^k * C(2*k, k)^2 * C(2*n+k, 2*n-k). - Paul D. Hanna, Aug 17 2014

From Peter Bala, Mar 14 2018: (Start)

a(n) = (-1)^n*P(2*n,sqrt(-3)), where P(n,x) denotes the n-th Legendre polynomial. See A008316.

a(n) = 1/C(2*n,n)*Sum_{k = 0..n} (-1)^(n+k)*C(n,k)*C(n+k,k)* C(2*n+2*k,n+k) = Sum_{k = 0..n} (-1)^(n+k)*C(2*k,k)*C(n,k) *C(2*n+2*k,2*n)/C(n+k,n). In general, P(2*n,sqrt(1+4*x)) = 1/C(2*n,n)*Sum_{k = 0..n} C(n,k)*C(n+k,k)*C(2*n+2*k,n+k) *x^k.

a(n) = Sum_{k = 0..2*n} C(2*n,k)^2 * u^(n-k), where u = (1 - sqrt(-3))/2 is a primitive sixth root of unity.

a(n) = (-1)^n*Sum_{k = 0..2*n} C(2*n,k)*C(2*n+k,k)*u^(2*k).

(End)

a(n) = (-1)^n*hypergeom([-n, n + 1/2], [1], 4). - Peter Luschny, Mar 16 2018

EXAMPLE

G.f.: A(x) = 1 + 5*x + 51*x^2 + 587*x^3 + 7123*x^4 + 89055*x^5 +...

where

A(x)^2 = (1-x + sqrt(1-14*x+x^2)) / (2*(1-14*x+x^2)).

Explicitly,

A(x)^2 = 1 + 10*x + 127*x^2 + 1684*x^3 + 22717*x^4 + 309214*x^5 + 4231675*x^6 + 58117672*x^7 + 800173945*x^8 +...+ A245923(n)*x^n +...

MAPLE

A245926 := n -> sqrt(add(binomial(4*n-2*k, 2*n-k)*binomial(4*n-k, k)^2, k=0..2*n)); seq(A245926(n), n=0..20); # Peter Luschny, Aug 17 2014

MATHEMATICA

CoefficientList[Series[Sqrt[(1 - x + Sqrt[1 - 14*x + x^2])/(2*(1 - 14*x + x^2))], {x, 0, 50}], x] (* G. C. Greubel, Jan 29 2017 *)

a[n_] := (-1)^n Hypergeometric2F1[-n, n + 1/2, 1, 4];

Table[a[n], {n, 0, 19}] (* Peter Luschny, Mar 16 2018 *)

PROG

(PARI) /* From definition: */

{a(n)=polcoeff( sqrt( (1-x + sqrt(1-14*x+x^2 +x*O(x^n))) / (2*(1-14*x+x^2 +x*O(x^n))) ), n)}

for(n=0, 20, print1(a(n), ", "))

(PARI) /* From formula for a(n)^2: */

{a(n)=sqrtint(sum(k=0, 2*n, sum(j=0, 4*n-2*k, (-1)^(j+k)*binomial(4*n-k, j+k)^2*binomial(j+k, k)^2)))}

for(n=0, 20, print1(a(n), ", "))

(PARI) /* From formula for a(n)^2: */

{a(n) = sqrtint( sum(k=0, 2*n, binomial(2*k, k)^2*binomial(2*n+k, 2*n-k)*(-1)^k) )}

for(n=0, 20, print1(a(n), ", "))

CROSSREFS

Cf. A245925, A245927, A245923, A243946.

Sequence in context: A041040 A223002 A180511 * A190734 A154886 A268138

Adjacent sequences:  A245923 A245924 A245925 * A245927 A245928 A245929

KEYWORD

nonn,easy

AUTHOR

Paul D. Hanna, Aug 15 2014

STATUS

approved

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Last modified September 19 14:43 EDT 2019. Contains 327198 sequences. (Running on oeis4.)