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A107877 Column 1 of triangle A107876. 9
1, 1, 2, 7, 37, 268, 2496, 28612, 391189, 6230646, 113521387, 2332049710, 53384167192, 1348601249480, 37291381915789, 1120914133433121, 36406578669907180, 1271084987848923282, 47487293697623885913, 1890771531272515677250, 79947079338974990793060 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Also number of subpartitions of partition consisting of first n-1 triangular numbers; e.g., a(4) = subp([1,3,6]) = 37. - Franklin T. Adams-Watters, Jun 26 2006

Number of length-n restricted growth strings (RGS) [s(0),s(1),...,s(n-1)] where s(0)=0 and s(k)<=s(k-1)+k, see Fxtbook link and example. [Joerg Arndt, Apr 30 2011]

Number of Dyck paths whose ascent lengths are exactly {1,2,..n+1}, for example the a(2) = 2 paths are uduuduuudddd and uduudduuuddd. [David Scambler, May 30 2012]

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..150

Joerg Arndt, Matters Computational (The Fxtbook), section 17.3.6, pp. 368-369

FORMULA

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

G.f.: 1 = Sum_{k>=0} a(k)*x^k/(1+x)^((k+1)*(k+2)/2).

From Benedict W. J. Irwin, Nov 26 2016: (Start)

Conjecture: a(n) can be expressed with a series of nested sums,

a(3) = Sum_{i=1..2} i+2,

a(4) = Sum_{i=1..2}Sum_{j=1..i+2} j+3,

a(5) = Sum_{i=1..2}Sum_{j=1..i+2}Sum_{k=1..j+3} k+4,

a(6) = Sum_{i=1..2}Sum_{j=1..i+2}Sum_{k=1..j+3}Sum_{l=1..k+4} l+5. (End)

EXAMPLE

1 = 1*(1-x)^1 + 1*x*(1-x)^2 + 2*x^2*(1-x)^4 + 7*x^3*(1-x)^7 + 37*x^4*(1-x)^11 + 268*x^5*(1-x)^16 + 2496*x^6*(1-x)^22 + ...

Also equals the final term in rows of the triangle where row n+1 equals the partial sums of row n with the final term repeated n+1 times, starting with a '1' in row 0, as illustrated by:

1;

1, 1;

1, 2,. 2,. 2;

1, 3,. 5,. 7,. 7,. 7,.. 7;

1, 4,. 9, 16, 23, 30,. 37,. 37,. 37,. 37,. 37;

1, 5, 14, 30, 53, 83, 120, 157, 194, 231, 268, 268, 268, 268, 268, 268; ...

Restricted growth strings: a(0)=1 corresponds to the empty string; a(1)=1 to [0];

a(2) = 2 to [00] and [01]; a(3)=7 to

  1:  [ 0 0 0 ],

  2:  [ 0 0 1 ],

  3:  [ 0 0 2 ],

  4:  [ 0 1 0 ],

  5:  [ 0 1 1 ],

  6:  [ 0 1 2 ],

  7:  [ 0 1 3 ].

[Joerg Arndt, Apr 30 2011]

MAPLE

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

      b(n-1, y+i-n), i=max(1, n-y)..n*(n-1)/2+1-y))

    end:

a:= n-> b(n+1, 0):

seq(a(n), n=0..25);  # Alois P. Heinz, Nov 26 2016

MATHEMATICA

a[ n_, k_: 1, j_: 1] := If[ n < 2, Boole[n >= 0], a[n, k, j] = Sum[a[n - 1, i, j + 1], {i, k + j}]]; (* Michael Somos, Nov 26 2016 *)

PROG

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

CROSSREFS

Cf. A107876, A107878, A107879.

Cf. A115728, A115729.

Sequence in context: A144301 A083659 A036247 * A001028 A116481 A102743

Adjacent sequences:  A107874 A107875 A107876 * A107878 A107879 A107880

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Jun 04 2005, Apr 10 2007

STATUS

approved

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Last modified March 27 22:02 EDT 2017. Contains 284182 sequences.