OFFSET
1,2
COMMENTS
"Ferrers matrix" is defined (A237981) as follows: an m X m matrix (x(i,j)) of 0's and 1's satisfying three properties: (1) x(1,m) = 1 or x(m,1) = 1; (2) x(i,j+1) >= x(i,j) for j=1..m-1 and i = 1..m; and (3) x(i+1,j) >= x(i,j) for i=1..m-1 and j=1..m. Ferrers matrices arise from Ferrers graphs of partitions, as follows: suppose that p is a partition of n, and let m = max{greatest part of p, number of parts of p}. Write the Ferrers graph of p with 1s as nodes, and pad the graph with 0's to form an m X m square matrix, which is the Ferrers matrix of p.
LINKS
Clark Kimberling and Peter J. C. Moses, Ferrers Matrices and Related Partitions of Integers
EXAMPLE
First 17 rows:
1
2
2 .... 1
4 .... 1
4 .... 3
8 .... 2 .... 1
10 ... 3 .... 2
14 ... 7 .... 1
20 ... 5 .... 5
30 ... 5 .... 6 ... 1
36 ... 15 ... 2 ... 3
52 ... 16 ... 6 ... 3
70 ... 13 ... 15 .. 3
94 ... 22 ... 12 .. 7
122 ... 32 .. 8 ... 13 .. 1
160 ... 45 .. 12 .. 10 .. 4
Row 5 counts 4 antidiagonals that have exactly one 1 and 3 antidiagonals that have exactly two 1's. The Ferrers matrix for each of the latter three cases are as shown below.
For the partition 32:
1 1 1
1 1 0
0 0 0 (antidiagonal, from row 1: 1,1,0)
For the partition 311:
1 1 1
1 0 0
1 0 0 (antidiagonal, from row 1: 1,0,1,)
For the partition 221:
1 1 0
1 1 0
1 0 0 (antidiagonal, from row 1: 0,1,1)
MATHEMATICA
z = 30; ferrersMatrix[list_] := PadRight[Map[Table[1, {#}] &, #], {#, #} &[Max[#, Length[#]]]] &[list]; diagAntidiagDots[list_] := {Total[Diagonal[#]], Total[Diagonal[Reverse[#]]]} &[ferrersMatrix[list]]; u[n_, k_] := Length[Select[ Map[diagAntidiagDots, IntegerPartitions[n]], #[[2]] == k &]]; t[n_] := t[n] = Floor[(-1 + Sqrt[1 + 8 n])/2]; w = Table[u[n, k], {n, 1, z}, {k, 1, t[n]}]; y = Flatten[w] (* A238606 *) (* Peter J. C. Moses, Mar 01 2014 *)
CROSSREFS
KEYWORD
nonn,tabf,easy
AUTHOR
Clark Kimberling, Mar 01 2014
STATUS
approved