OFFSET
0,3
LINKS
Sean A. Irvine, Table of n, a(n) for n = 0..999
G. Kreweras, Les préordres totaux compatibles avec un ordre partiel, Math. Sci. Humaines No. 53 (1976), 5-30. (The numbers u_r^n on page 20.)
G. Kreweras, Les préordres totaux compatibles avec un ordre partiel, Math. Sci. Humaines No. 53 (1976), 5-30. (Annotated scanned copy)
FORMULA
From Sean A. Irvine, Mar 12 2017: (Start)
A079502 can be constructed one row at a time from the corresponding row of A050447. For row n, construct up to the n-th difference sequence of row n in A050447, retaining the first element of each difference sequence. Row n of A079502 is then constructed backwards (i.e., starting with A079502(n,n) and computing down to A079502(n,2)) from the first element of the n-th difference sequence, then successively subtracting the first element of the previous difference sequences. More precisely, let R_n denote the n-th row of A050447 augmented with R_n(1) = 0, and R_n^(d) the d-th difference of that row, such that R_n^(0)(m) = R_n(m) and R_n^(k)(m) = R_n^(k-1)(m+1) - R_n^(k-1)(m). Row n of A079502 is then T(n,n) = R_n^(n)(0) and for m < n, T(n,m) = R_n^(n)(0) - T(n,m+1).
For example, starting with row 4 of A050447: [0], 1, 8, 31, 85, 190, 371, ..., we construct up to order 4 difference sequences: first-differences 1, 7, 23, 54, 105, 181, ...; second-differences 6, 16, 31, 51, 76, ...; third-differences 10, 15, 20, 25, ...; fourth-differences 5, 5, 5, ... (constant). Only the first elements of these difference sequences are needed. Thus T(4,4) = 5, T(4,3) = 10 - 5 = 5, T(4,2) = 6 - (10 - 5) = 1, T(4,1) = 1 - (6 - (10 - 5)) = 0. (End)
EXAMPLE
Triangle T(n,k) begins:
1;
1, 2;
1, 5, 5;
1, 10, 24, 16;
1, 18, 79, 122, 61;
1, 31, 223, 602, 680, 272;
1, 52, 579, 2439, 4682, 4155, 1385;
1, 86, 1432, 8856, 25740, 38072, 27776, 7936;
MATHEMATICA
t[n_, m_] := t[n, m] = If[m == 0, 1, t[n, m - 1] + Sum[t[2 k, m - 1] t[n - 1 - 2 k, m], {k, 0, (n - 1)/2}]]; Map[Function[s, Rest@ Reverse@ Map[Abs@ Fold[#2 - #1 &, Reverse@ Take[s, #]] &, Range@ Length@ s]]@ Reverse@ Map[First, NestList[Differences@ # &, {First@ #}~Join~Differences@ #, Length@ # - 2]] &, Table[t[n, k], {n, 2, 11}, {k, 0, n}]] (* Michael De Vlieger, Mar 13 2017, after Jean-François Alcover at A050447 *)
CROSSREFS
KEYWORD
AUTHOR
N. J. A. Sloane, Jan 21 2003
EXTENSIONS
More terms from Sean A. Irvine, Mar 12 2017
STATUS
approved