

A141058


Pats by first entry.


0



1, 0, 1, 0, 1, 1, 0, 1, 2, 2, 0, 2, 3, 4, 5, 0, 5, 6, 7, 10, 14, 0, 14, 15, 15, 18, 28, 42, 0, 42, 42, 38, 40, 51, 84, 132, 0, 132, 126, 107, 103, 115, 154, 264, 429, 0, 429, 396, 322, 292, 299, 350, 486, 858, 1430, 0, 1430, 1287, 1014, 882, 852, 915, 1110, 1584, 2860, 4862
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OFFSET

0,9


COMMENTS

T(n,k) is the number of pats on [0,n] with first entry k. Pats are defined recursively in the Oakley/Wisner reference. Briefly, a oneentry permutation is a pat and a twoormoreentry permutation p on any set of integers is a pat iff (i) there is a unique way to split p as the concatenation of nonempty permutations p_1 and p_2 such that all entries in p_1 exceed all entries in p_2, and (ii) reverse(p1) and reverse(p2) are pats. Thus 21 and 43 are pats but 12 is not and p = 3412 is a pat using p1 = 34 and p2 = 12. Pats on [1,n+1] (considered by Oakley/Wisner in the definition of flexagons) correspond to pats on [0,n] by subtracting 1 from each entry.
Also, pats on [0,n] with last entry k correspond to pats with first entry nk under the reversecomplement operation on permutations.


LINKS

Table of n, a(n) for n=0..65.
C. O. Oakley and R. J. Wisner, Flexagons, Amer. Math. Monthly 64 (1957), 143154.


FORMULA

G.f.: Sum_{n>=0,k>=0} T(n,k)*x^n*y^k = (1 + x * y * CatalanGF(x * y))/(1  x^2 * y * CatalanGF(x) * CatalanGF(x * y)) where CatalanGF(x) = (1sqrt(14*x))/(2*x) is the g.f. for the Catalan numbers A000108.


EXAMPLE

T(4,2)=3 counts 24301, 23140, 21430.
Table begins
1
0...1
0...1...1
0...1...2...2
0...2...3...4...5
0...5...6...7..10..14
0..14..15..15..18..28..42


MATHEMATICA

a[0, 0]=1; a[n_, k_]/; n>=1 && 0<=k<=n := a[n, k] = (* count by splitting point in condition (i) *) Sum[a[i, nk]CatalanNumber[ni1], {i, nk, n1}]; Table[a[n, k], {n, 0, 10}, {k, 0, n}]


CROSSREFS

The Catalan numbers A000108 appear as row sums and in the second column and on the main diagonal.
Sequence in context: A072738 A165316 A215976 * A102706 A105673 A259761
Adjacent sequences: A141055 A141056 A141057 * A141059 A141060 A141061


KEYWORD

nonn,tabl


AUTHOR

David Callan, Aug 01 2008


STATUS

approved



