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A106336 Number of ways of writing n as the sum of n+1 triangular numbers, divided by n+1. 7
1, 1, 1, 2, 5, 11, 25, 64, 169, 442, 1172, 3180, 8730, 24116, 67159, 188568, 532741, 1512695, 4315996, 12369324, 35587923, 102747636, 297601382, 864525312, 2518185362, 7353088206, 21520084301, 63115752910, 185474840912, 546042990300, 1610314638958 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

Apparently: Number of Dyck n-paths with each ascent length being a triangular number. - David Scambler, May 09 2012

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..2000

FORMULA

G.f.: A(x) = (1/x)*serreverse( x*eta(x)/eta(x^2)^2 ).

G.f. satisfies:

(1) A(x) = F(x*A(x)) where F(x) = Sum_{n>=0} x^(n*(n+1)/2).

(2) log(A(x)) = Sum_{n>=1} A106337(n)/n*x^n.

(3) A(x) = Product_{n>=1} (1 + (x*A(x))^n)*(1 - (x*A(x))^(2n)). - Paul D. Hanna, Oct 23 2010

(4) A(x) = exp( Sum_{n>=1} (x^n*A(x)^n/(1 + x^n*A(x)^n))/n ). - Paul D. Hanna, Jun 01 2011

EXAMPLE

G.f.: A(x) = 1 + x + x^2 + 2*x^3 + 5*x^4 + 11*x^5 + 25*x^6 + 64*x^7 +...

A(x) = F(x*A(x)) where F(x) = 1 + x + x^3 + x^6 + x^10 + x^15 + x^21 + ...

The radius of convergence equals r = 0.322627632692191133... (A106335)

at which the g.f. converges to A(r) = 1.987369721184684145... (A106334).

MAPLE

b:= proc(n) option remember; expand(`if`(n=0, 1,

      add(`if`(issqr(8*j+1), x*b(n-j), 0), j=1..n)))

    end:

a:= n-> (p-> add(coeff(p, x, i)*binomial(1+n, i),

             i=0..n)/(n+1))(b(n)):

seq(a(n), n=0..35);  # Alois P. Heinz, Jul 31 2017

MATHEMATICA

f[x_, y_, d_] := f[x, y, d] = If[x < 0 || y < x, 0, If[x == 0 && y == 0, 1, f[x-1, y, 0] + f[x, y - If[d == 0, 1, Ceiling[Sqrt[2*d]]], If[d == 0, 1, Ceiling[Sqrt[2*d]] + d]]]]; Table[f[n, n, 0], {n, 0, 30}] (* David Scambler, May 09 2012 *)

PROG

(PARI) {a(n)=local(X); if(n<0, 0, X=x+x*O(x^n); polcoeff(eta(X^2)^(2*n+2)/eta(X)^(n+1)/(n+1), n))}

(PARI) {a(n)=if(n<0, 0, polcoeff( sum(k=1, (sqrtint(8*n+1)+1)\2, x^((k^2-k)/2), x*O(x^n))^(n+1)/(n+1), n))}

(PARI) {a(n)=local(A=1+x+x*O(x^n)); for(i=1, n, A=prod(m=1, n, (1+(x*A)^m)*(1-(x*A)^(2*m)))); polcoeff(A, n)} \\ Paul D. Hanna, Oct 23 2010

(PARI) {a(n)=local(A=1+x); for(i=1, n, A=exp(sum(m=1, n, (x*A)^m/(1+(x*A)^m+x*O(x^n))/m))); polcoeff(A, n)} \\ Paul D. Hanna, Jun 01 2011

CROSSREFS

Cf. A106333, A106334, A106335, A106337; related: A109085.

Sequence in context: A239812 A097779 A319768 * A226974 A047775 A001432

Adjacent sequences:  A106333 A106334 A106335 * A106337 A106338 A106339

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Apr 29 2005

EXTENSIONS

Edited by Paul D. Hanna, Jun 01 2011

STATUS

approved

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Last modified September 23 16:01 EDT 2020. Contains 337310 sequences. (Running on oeis4.)