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 n-1 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
Alois P. Heinz, Table of n, a(n) for n = 1..80
P. Erdős, R. K. Guy and J. W. Moon, On refining partitions, J. London Math. Soc., 9 (1975), 565-570.
R. K. Guy, Letter to N. J. A. Sloane, June 24 1971: front, back [Annotated scanned copy, with permission]
Olivier Gérard, The ranked posets L(2),...,L(8)
Gus Wiseman, Hasse Diagrams of Partition Refinement Posets n=1..9
Gus Wiseman, Hasse Diagrams of Partition Refinement Posets n=1..9, Version 1, [Cached copy, with permission]
Gus Wiseman, Hasse Diagrams of Partition Refinement Posets n=1..9, Version 2, [Cached 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[i-1], 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]):
seq(a(n), n=1..30); # Alois P. Heinz, Sep 22 2019
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
N. J. A. Sloane. Entry revised by N. J. A. Sloane, Jun 11 2012
EXTENSIONS
a(17)-a(25) from Mitch Harris, Jan 19 2006
STATUS
approved