A247031 G.f.: 1 = Sum_{n>=0} a(n)*x^n * [ Sum_{k=0..n+1} binomial(n+1, k)^3 * (-x)^k ]^3.

1, 3, 69, 5005, 806148, 239220375, 116532061510, 86173621173099, 91417549409916684, 133300597778263476112, 258360728839130761571757, 647880493609691058921741273, 2055869510173976408422116133220, 8103111707775918586405906798540650, 39047811321420953231675462397758519802

Offset

0,2

Comments

Compare to a g.f. of A006632(n) = 3*binomial(4*n+3,n)/(4*n+3):

1 = Sum_{n>=0} A006632(n)*x^n * [ Sum_{k=0..n+1} binomial(n+1, k)*(-x)^k ]^3

where A006632 equals the self-convolution cube of A002293.

...

a(3*n) == 2 (mod 3) iff A245658(n) == 2 (mod 3), where A245658 is the self-convolution cube root of this sequence (conjecture).

Links

Paul D. Hanna, Table of n, a(n) for n = 0..200

Example

O.g.f.: A(x) = 1 + 3*x + 69*x^2 + 5005*x^3 + 806148*x^4 + 239220375*x^5 +...
such that the coefficients satisfy:
1 = 1*(1-x)^3 + 3*x*(1-2^3*x+x^2)^3 + 69*x^2*(1-3^3*x+3^3*x^2-x^3)^3 + 5005*x^3*(1-4^3*x+6^3*x^2-4^3*x^3+x^4)^3 + 806148*x^4*(1-5^3*x+10^3*x^2-10^3*x^3+5^3*x^4-x^5)^3 +...
Note that the cube-root of the o.g.f., A(x)^(1/3), is an integer series:
A(x)^(1/3) = 1 + x + 22*x^2 + 1624*x^3 + 264962*x^4 + 79136637*x^5 + 38671111558*x^6 + 28642761340956*x^7 + 30413158977739302*x^8 +...+ A245658(n)*x^n +...
CONJECTURE: given
G(x,m,j) = Sum_{n>=0} a(n) * (m*x)^n * [ Sum_{k=0..n+1} C(n+1, k)^3 * (j*x)^k ]^3
then G(x,m,j)^(1/3) is an integer series in x whenever m, j, are integers.
OBSERVATIONS.
The terms of this sequence are congruent to 2 modulo 3 at positions:
[12, 36, 93, 108, 111, 114, 120, 174, 255, 279, ...].
The terms of A245658 are congruent to 2 modulo 3 at positions:
[4, 12, 31, 36, 37, 38, 40, 58, 85, 93, ...].

Prog

(PARI) {a(n)=if(n==0, 1, -polcoeff(sum(m=0, n-1, a(m)*x^m*sum(k=0, m+1, binomial(m+1, k)^3 * (-x)^k +x*O(x^n) )^3 ), n))}
for(n=0, 20, print1(a(n), ", "))

Crossrefs

Cf. A245658, A247030, A212370.

Sequence in context: A012096 A012074 A350720 * A037109 A304146 A305486

Adjacent sequences: A247028 A247029 A247030 * A247032 A247033 A247034

Keyword

nonn

Author

Paul D. Hanna, Sep 09 2014

Status

approved