

A002846


Number of ways of transforming a set of n indistinguishable objects into n singletons via a sequence of n1 refinements.
(Formerly M1251 N0478)


43



1, 1, 1, 2, 4, 11, 33, 116, 435, 1832, 8167, 39700, 201785, 1099449, 6237505, 37406458, 232176847, 1513796040, 10162373172, 71158660160, 511957012509, 3819416719742, 29195604706757, 230713267586731, 1861978821637735, 15484368121967620, 131388840051760458
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OFFSET

1,4


COMMENTS

Construct the ranked poset L(n) whose nodes are the A000041(n) partitions of n, with all the partitions into the same number of parts having the same rank. A partition into k parts is joined to a partition into k+1 parts if the latter is a refinement of the former.
The partition n^1 is at the left and the partition 1^n at the right. The illustration by Olivier Gérard shows the posets L(2) through L(8).
Then a(n) is the number of paths of length n1 in L(n) that join n^1 to 1^n.
Stated another way, a(n) is the number of maximal chains in the ranked poset L(n). (This poset is not a lattice for n > 4.)  Comments corrected by Gus Wiseman, May 01 2016


REFERENCES

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS

R. K. Guy, Letter to N. J. A. Sloane, June 24 1971: front, back [Annotated scanned copy, with permission]


EXAMPLE

a(5) = 4 because there are 4 paths from top to bottom in this lattice:
.
ooooo
/ \
o.oooo oo.ooo
 X 
o.o.ooo o.oo.oo
\ /
o.o.o.oo

o.o.o.o.o
.
(This is the ranked poset L(5), but drawn vertically rather than horizontally.)


MAPLE

v:= l> [seq(`if`(i=1 or l[i]>l[i1], seq(subs(1=[][], sort(subsop(
i=[j, l[i]j][], l))), j=1..l[i]/2), [][]), i=1..nops(l))]:
b:= proc(l) option remember; `if`(max(l)<2, 1, add(b(h), h=v(l))) end:
a:= n> b([n]):


MATHEMATICA

<<posets.m Table[Build[NumP[n], np]; Last@MaximalChainsDown@np, {n, 1, 25}] (* Mitch Harris, Jan 19 2006 *)


PROG

(Sage) def A002846(n): return Posets.IntegerPartitions(n).chain_polynomial().leading_coefficient() # Max Alekseyev, Dec 23 2015


CROSSREFS



KEYWORD

nonn,nice


AUTHOR



EXTENSIONS



STATUS

approved



