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A322243 a(n) = A322242(n)^2, the square of the central coefficient in (1 + 3*x + 4x^2)^n. 4
1, 9, 289, 9801, 370881, 14768649, 609546721, 25795893321, 1112301387649, 48661046208009, 2153525838773409, 96206353829163081, 4331637064535243841, 196320612369490652169, 8948100956521251936609, 409841929215895450531401, 18851871634769751620818689, 870412126485504031282344969, 40322236319029726815932366881, 1873525179289326411511891685961, 87284881539374630658475997415361 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

The g.f. of A322242 is 1/sqrt(1 - 6*x - 7*x^2).

LINKS

Robert Israel, Table of n, a(n) for n = 0..593

FORMULA

G.f.: 1 / AGM(1 + 7*x, sqrt((1 - x)*(1 - 49*x)) ), where AGM(x,y) = AGM((x+y)/2, sqrt(x*y)) is the arithmetic-geometric mean.

G.f.: 1 / AGM((1-x)*(1-7*x), (1+x)*(1+7*x)) = Sum_{n>=0} a(n)*x^(2*n).

a(n) = A322242(n)^2 where A322242(n) = Sum_{k=0..n} (-1)^(n-k) * 2^k * binomial(n,k)*binomial(2*k,k).

343*(5+2*n)*(n+1)^2*a(n)-7*(3+2*n)*(43*n^2+172*n+163)*a(n+1)-(5+2*n)*(43*n^2+172*n+163)*a(n+2)+(3+2*n)*(n+3)^2*a(n+3)=0. - Robert Israel, Dec 10 2018

a(n) ~ 7^(2*n + 1) / (8*Pi*n). - Vaclav Kotesovec, Sep 27 2019

EXAMPLE

G.f.: A(x) = 1 + 9*x + 289*x^2 + 9801*x^3 + 370881*x^4 + 14768649*x^5 + 609546721*x^6 + 25795893321*x^7 + 1112301387649*x^8 + 48661046208009*x^9 + ...

such that

A(x) = 1 + 3^2*x + 17^2*x^2 + 99^2*x^3 + 609^2*x^4 + 3843^2*x^5 + 24689^2*x^6 + 160611^2*x^7 + 1054657^2*x^8 + 6975747^2*x^9 + ... + A322242(n)^2*x^n + ...

MAPLE

f:= gfun:-rectoproc({343*(5+2*n)*(n+1)^2*a(n)-7*(3+2*n)*(43*n^2+172*n+163)*a(n+1)-(5+2*n)*(43*n^2+172*n+163)*a(n+2)+(3+2*n)*(n+3)^2*a(n+3)=0, a(0)=1, a(1)=3^2, a(2)=17^2}, a(n), remember):

map(f, [$0..30]); # Robert Israel, Dec 10 2018

MATHEMATICA

f[n_] := (CoefficientList[Expand[(1 + 3*x + 4*x^2)^n], x][[n + 1]])^2; Array[f, 22, 0] (* Amiram Eldar, Dec 10 2018 *)

CoefficientList[Series[2*EllipticK[1 - (1 + 7*x)^2/((1 - 49*x)*(1 - x))] / (Pi*Sqrt[(1 - 49*x)*(1 - x)]), {x, 0, 20}], x] (* Vaclav Kotesovec, Sep 27 2019 *)

PROG

(PARI) /* a(n) = A322242(n)^2 */

{a(n)=polcoeff(1/sqrt(1 - 6*x - 7*x^2 +x*O(x^n)), n)^2}

for(n=0, 30, print1(a(n), ", "))

(PARI) /* Using AGM: */

{a(n)=polcoeff( 1 / 1 / agm(1 + 7*x, sqrt((1 - x)*(1 - 7^2*x) +x*O(x^n))), n)}

for(n=0, 30, print1(a(n), ", "))

CROSSREFS

Cf. A322242.

Sequence in context: A078326 A118893 A055792 * A053935 A332129 A086699

Adjacent sequences:  A322240 A322241 A322242 * A322244 A322245 A322246

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Dec 08 2018

STATUS

approved

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Last modified August 11 18:37 EDT 2020. Contains 336428 sequences. (Running on oeis4.)