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 A137251 Triangle T(n,k) read by rows: number of k X k triangular matrices with nonnegative integer entries and without zero rows or columns such that sum of all entries is equal to n, n>=1, 1<=k<=n. 12
 1, 1, 1, 1, 3, 1, 1, 6, 7, 1, 1, 10, 26, 15, 1, 1, 15, 71, 98, 31, 1, 1, 21, 161, 425, 342, 63, 1, 1, 28, 322, 1433, 2285, 1138, 127, 1, 1, 36, 588, 4066, 11210, 11413, 3670, 255, 1, 1, 45, 1002, 10165, 44443, 79781, 54073, 11586, 511, 1, 1, 55, 1617, 23056, 150546, 434638, 528690, 246409, 36038, 1023, 1 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS Row sums are A022493. Number of ascent sequences of length n with k-1 ascents, see example. [Joerg Arndt, Nov 03 2012] Number of interval orders on n elements having exactly k maximal antichains. Also, number of interval orders on n elements having an interval representation with k distinct endpoints, but not with k-1 distinct endpoints. Also, number of interval orders on n elements whose elements define k distinct strict down-sets (a strict down-set defined by an element x of a poset (P,<) is the set {y in P: y=1} T(n,k)x^n y^k = Sum_{n>=1} y^n Prod_{i=1..n} (1-(1-x)^i)/(y+(1-x)^i-y*(1-x)^i). See Jelínek's paper, Corollary 2.5. - Vít Jelínek, Sep 06 2014 EXAMPLE Triangle starts: 01:  1, 02:  1, 1, 03:  1, 3, 1, 04:  1, 6, 7, 1, 05:  1, 10, 26, 15, 1, 06:  1, 15, 71, 98, 31, 1, 07:  1, 21, 161, 425, 342, 63, 1, 08:  1, 28, 322, 1433, 2285, 1138, 127, 1, 09:  1, 36, 588, 4066, 11210, 11413, 3670, 255, 1, 10:  1, 45, 1002, 10165, 44443, 79781, 54073, 11586, 511, 1, 11:  1, 55, 1617, 23056, 150546, 434638, 528690, 246409, 36038, 1023, 1, 12:  1, 66, 2497, 48400, 451515, 1968580, 3895756, 3316193, 1090517, 110930, 2047, 1, ... From Joerg Arndt, Nov 03 2012: (Start) The 53 ascent sequences of length 5 together with their numbers of ascents are (dots for zeros): 01:  [ . . . . . ]   0      28:  [ . 1 1 . 1 ]   2 02:  [ . . . . 1 ]   1      29:  [ . 1 1 . 2 ]   2 03:  [ . . . 1 . ]   1      30:  [ . 1 1 1 . ]   1 04:  [ . . . 1 1 ]   1      31:  [ . 1 1 1 1 ]   1 05:  [ . . . 1 2 ]   2      32:  [ . 1 1 1 2 ]   2 06:  [ . . 1 . . ]   1      33:  [ . 1 1 2 . ]   2 07:  [ . . 1 . 1 ]   2      34:  [ . 1 1 2 1 ]   2 08:  [ . . 1 . 2 ]   2      35:  [ . 1 1 2 2 ]   2 09:  [ . . 1 1 . ]   1      36:  [ . 1 1 2 3 ]   3 10:  [ . . 1 1 1 ]   1      37:  [ . 1 2 . . ]   2 11:  [ . . 1 1 2 ]   2      38:  [ . 1 2 . 1 ]   3 12:  [ . . 1 2 . ]   2      39:  [ . 1 2 . 2 ]   3 13:  [ . . 1 2 1 ]   2      40:  [ . 1 2 . 3 ]   3 14:  [ . . 1 2 2 ]   2      41:  [ . 1 2 1 . ]   2 15:  [ . . 1 2 3 ]   3      42:  [ . 1 2 1 1 ]   2 16:  [ . 1 . . . ]   1      43:  [ . 1 2 1 2 ]   3 17:  [ . 1 . . 1 ]   2      44:  [ . 1 2 1 3 ]   3 18:  [ . 1 . . 2 ]   2      45:  [ . 1 2 2 . ]   2 19:  [ . 1 . 1 . ]   2      46:  [ . 1 2 2 1 ]   2 20:  [ . 1 . 1 1 ]   2      47:  [ . 1 2 2 2 ]   2 21:  [ . 1 . 1 2 ]   3      48:  [ . 1 2 2 3 ]   3 22:  [ . 1 . 1 3 ]   3      49:  [ . 1 2 3 . ]   3 23:  [ . 1 . 2 . ]   2      50:  [ . 1 2 3 1 ]   3 24:  [ . 1 . 2 1 ]   2      51:  [ . 1 2 3 2 ]   3 25:  [ . 1 . 2 2 ]   2      52:  [ . 1 2 3 3 ]   3 26:  [ . 1 . 2 3 ]   3      53:  [ . 1 2 3 4 ]   4 27:  [ . 1 1 . . ]   1 There is 1 ascent sequence with no ascent, 10 with one ascent, etc., giving the fourth row [1, 10, 26, 15, 1]. (End) MAPLE b:= proc(n, i, t) option remember; local j; if n<1 then [0\$t, 1]       else []; for j from 0 to t+1 do zip((x, y)->x+y, %,       b(n-1, j, t+`if`(j>i, 1, 0)), 0) od; % fi     end: T:= n-> b(n-1, 0, 0)[]: seq(T(n), n=1..12);  # Alois P. Heinz, May 20 2013 MATHEMATICA zip[f_, x_List, y_List, z_] := With[{m = Max[Length[x], Length[y]]}, f[PadRight[x, m, z], PadRight[y, m, z]]]; b[n_, i_, t_] := b[n, i, t] = Module[{j, pc}, If[n<1, Append[Array[0&, t], 1], pc = {}; For[j = 0, j <= t+1, j++, pc = zip[Plus, pc, b[n-1, j, t+If[j>i, 1, 0]], 0]]; pc]]; T[n_] := b[n-1, 0, 0]; Table[T[n], {n, 1, 12}] // Flatten (* Jean-François Alcover, Jan 29 2014, after Alois P. Heinz *) CROSSREFS Cf. A022493 (number of ascent sequences), A218577 (ascent sequences with maximal element k), A175579 (ascent sequences with k zeros). Sequence in context: A008278 A213735 A056858 * A158359 A046716 A202605 Adjacent sequences:  A137248 A137249 A137250 * A137252 A137253 A137254 KEYWORD nonn,tabl AUTHOR Vladeta Jovovic, Mar 11 2008 STATUS approved

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Last modified October 20 10:31 EDT 2019. Contains 328257 sequences. (Running on oeis4.)