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A115720 Triangle T(n,k) is the number of partitions of n with Durfee square k. 25
1, 0, 1, 0, 2, 0, 3, 0, 4, 1, 0, 5, 2, 0, 6, 5, 0, 7, 8, 0, 8, 14, 0, 9, 20, 1, 0, 10, 30, 2, 0, 11, 40, 5, 0, 12, 55, 10, 0, 13, 70, 18, 0, 14, 91, 30, 0, 15, 112, 49, 0, 16, 140, 74, 1, 0, 17, 168, 110, 2, 0, 18, 204, 158, 5, 0, 19, 240, 221, 10, 0, 20, 285, 302, 20, 0, 21, 330, 407 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

T(n,k) is number of partitions of n-k^2 into parts of 2 kinds with at most k of each kind.

LINKS

Alois P. Heinz, Rows n = 0..600, flattened

Findstat, St000183: The side length of the Durfee square of an integer partition

Eric Weisstein's World of Mathematics, Durfee Square

FORMULA

T(n,k) = Sum_{i=0..n-k^2} P*(i,k)*P*(n-k^2-i), where P*(n,k) = P(n+k,k) is the number of partitions of n objects into at most k parts.

EXAMPLE

Triangle starts:

  1;

  0,  1;

  0,  2;

  0,  3;

  0,  4,  1;

  0,  5,  2;

  0,  6,  5;

  0,  7,  8;

  0,  8, 14;

  0,  9, 20,  1;

  0, 10, 30,  2;

From Gus Wiseman, Apr 12 2019: (Start)

Row n = 9 counts the following partitions:

  (9)          (54)       (333)

  (81)         (63)

  (711)        (72)

  (6111)       (432)

  (51111)      (441)

  (411111)     (522)

  (3111111)    (531)

  (21111111)   (621)

  (111111111)  (3222)

               (3321)

               (4221)

               (4311)

               (5211)

               (22221)

               (32211)

               (33111)

               (42111)

               (222111)

               (321111)

               (2211111)

(End)

MAPLE

b:= proc(n, i) option remember;

      `if`(n=0, 1, `if`(i<1, 0, b(n, i-1)+`if`(i>n, 0, b(n-i, i))))

    end:

T:= (n, k)-> add(b(m, k)*b(n-k^2-m, k), m=0..n-k^2):

seq(seq(T(n, k), k=0..floor(sqrt(n))), n=0..30); # Alois P. Heinz, Apr 09 2012

MATHEMATICA

b[n_, i_] := b[n, i] = If[n==0, 1, If[i<1, 0, b[n, i-1] + If[i>n, 0, b[n-i, i]]]]; T[n_, k_] := Sum[b[m, k]*b[n-k^2-m, k], {m, 0, n-k^2}]; Table[ T[n, k], {n, 0, 30}, {k, 0, Sqrt[n]}] // Flatten (* Jean-Fran├žois Alcover, Dec 03 2015, after Alois P. Heinz *)

durf[ptn_]:=Length[Select[Range[Length[ptn]], ptn[[#]]>=#&]];

Table[Length[Select[IntegerPartitions[n], durf[#]==k&]], {n, 0, 10}, {k, 0, Sqrt[n]}] (* Gus Wiseman, Apr 12 2019 *)

CROSSREFS

For a version without zeros see A115994. Row lengths are A003059. Row sums are A000041. Column k = 2 is A006918. Column k = 3 is A117485.

Cf. A008284, A115721, A115722, A257990, A325164.

Related triangles are A096771, A325188, A325189, A325192, with Heinz-encoded versions A263297, A325169, A065770, A325178.

Sequence in context: A073739 A223707 A046767 * A053120 A336836 A284976

Adjacent sequences:  A115717 A115718 A115719 * A115721 A115722 A115723

KEYWORD

nonn,tabf

AUTHOR

Franklin T. Adams-Watters, Mar 11 2006

STATUS

approved

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Last modified September 30 14:57 EDT 2020. Contains 337439 sequences. (Running on oeis4.)