1,2

Number of partitions with Ferrers plots that fit inside an n X n box, but not in an n-1 X n-1 box. - Wouter Meeussen (wouter.meeussen(AT)pandora.be), Dec 10 2001

From Benoit Cloitre, Jan 29 2002: Let m(1,j)=j, m(i,1)=i and m(i,j)=m(i- 1,j)+m(i,j-1); then a(n) = m(n,n):

1 2 3 4 .....

2 4 7 11 .......

3 7 14 25 .......

4 11 25 50 .......

This sequence also gives the number of clusters and non-crossing partitions of type D_n. - Frederic Chapoton (fchapoton(AT)voila.fr), Jan 31 2005

If Y is a 2-subset of a 2n-set X then a(n) is the number of (n+1)-subsets of X intersecting Y. - Milan R. Janjic (agnus(AT)blic.net), Nov 18 2007

Fomin, Sergey and Zelevinsky, Andrei, Y-systems and generalized associahedra, Ann. of Math. (2) 158,2003.

Hugh Thomas, math.CO/0311334: Tamari Lattices and Non-Crossing Partitions in Types B and D.

F. Chapoton, Clusters.

Milan Janjic, Two Enumerative Functions

a(n+1)=binomial(2*n, n)+2*sum(i=0, n-1, binomial(n+i, i) (V's in Pascal's Triangle) Jon Perry (perry(AT)globalnet.co.uk) Apr 13 2004

a(n) = n*C(n-1) - (n-1)*C(n-2), where C(n) = A000108(n) = Catalan(n). For example, a(5) = 50 = 5*C(4) - 4*C(3) - 5*14 - 3*5 = 70 - 20. Also equals A128064 * A000108. [That cannot be correct, since A128064 has several negative entries?] - Gary W. Adamson (qntmpkt(AT)yahoo.com), Dec 28 2007

Sum of 3 central terms of Pascal's triangle: 2*C(2+2*n, n)+C(2+2*n, 1+n). - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Dec 20 2005

a(n+1)=A051597(2n,n) . - Philippe DELEHAM (kolotoko(AT)wanadoo.fr), Nov 26 2006

Sums of {1}, {2, 1, 1}, {2, 2, 3, 3, 2, 1, 1}, {2, 2, 4, 5, 7, 6, 7, 5, 5, 3, 2, 1, 1}, ...

C:=proc(n) options operator, arrow: binomial(2*n, n)/(n+1) end proc: seq(n*C(n-1)-(n-1)*C(n-2), n=2..25); - Emeric Deutsch (deutsch(AT)duke.poly.edu), Jan 08 2008

Z:=(1-z-sqrt(1-4*z))/sqrt(1-4*z): Zser:=series(Z, z=0, 32): seq(coeff(Zser, z, n), n=1..24); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jan 01 2007

Left-central elements of the (1,2)-Pascal triangle A029635.

Cf. A000108, A024482 (diagonal from 2), A076540 (diagonal from 3), A000124 (row from 2), A004006 (row from 3), A006522 (row from 4).

Cf. A128064.

Sequence in context: A047065 A055990 A087945 this_sequence A076024 A062807 A117421

Adjacent sequences: A051921 A051922 A051923 this_sequence A051925 A051926 A051927

easy,nice,nonn

Barry E. Williams, Dec 19 1999

Edited by njas, May 03 2008, at the suggestion of R. J. Mathar.