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A004250
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Number of partitions of n into 3 or more parts.
(Formerly M1046)
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37
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0, 0, 1, 2, 4, 7, 11, 17, 25, 36, 50, 70, 94, 127, 168, 222, 288, 375, 480, 616, 781, 990, 1243, 1562, 1945, 2422, 2996, 3703, 4550, 5588, 6826, 8332, 10126, 12292, 14865, 17958, 21618, 25995, 31165, 37317, 44562
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OFFSET
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1,4
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COMMENTS
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Number of (n+1)-vertex spider graphs: trees with n+1 vertices and exactly 1 vertex of degree at least 3 (i.e. branching vertex). There is a trivial bijection with the objects described in the definition. - Emeric Deutsch, Feb 22 2014
Also the number of graphical partitions of 2n into n parts. - Gus Wiseman, Jan 08 2021
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
P. R. Stein, On the number of graphical partitions, pp. 671-684 of Proc. 9th S-E Conf. Combinatorics, Graph Theory, Computing, Congr. Numer. 21 (1978).
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LINKS
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FORMULA
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Let P(n,i) denote the number of partitions of n into i parts. Then a(n) = Sum_{i=3..n} P(n,i). - Thomas Wieder, Feb 01 2007
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EXAMPLE
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a(6)=7 because there are three partitions of n=6 with i=3 parts: [4, 1, 1], [3, 2, 1], [2, 2, 2] and two partitions with i=4 parts: [3, 1, 1, 1], [2, 2, 1, 1] and one partition with i=5 parts: [2, 1, 1, 1, 1]] and one partition with i=6 parts: [1, 1, 1, 1, 1, 1].
The a(3) = 1 through a(7) = 11 graphical partitions of 2n into n parts:
(222) (2222) (22222) (222222) (2222222)
(3221) (32221) (322221) (3222221)
(33211) (332211) (3322211)
(42211) (333111) (3332111)
(422211) (4222211)
(432111) (4322111)
(522111) (4331111)
(4421111)
(5222111)
(5321111)
(6221111)
(End)
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MAPLE
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with(combinat);
for i from 1 to 15 do pik(i, 3) od;
pik:= proc(n::integer, k::integer)
local i, Liste, Result;
if k > n or n < 0 or k < 1 then
return fail
end if;
Result := 0;
for i from k to n do
Liste:= PartitionList(n, i);
#print(Liste);
Result := Result + nops(Liste);
end do;
return Result;
end proc;
PartitionList := proc (n, k)
# Authors: Herbert S. Wilf and Joanna Nordlicht. Source: Lecture Notes
# "East Side West Side, ..." University of Pennsylvania, USA, 2002.
# Available at: http://www.cis.upenn.edu/~wilf/lecnotes.html
# Calculates the partition of n into k parts.
# E.g. PartitionList(5, 2) --> [[4, 1], [3, 2]].
local East, West;
if n < 1 or k < 1 or n < k then
RETURN([])
elif n = 1 then
RETURN([[1]])
else if n < 2 or k < 2 or n < k then
West := []
else
West := map(proc (x) options operator, arrow;
[op(x), 1] end proc, PartitionList(n-1, k-1)) end if;
if k <= n-k then
East := map(proc (y) options operator, arrow;
map(proc (x) options operator, arrow; x+1 end proc, y) end proc, PartitionList(n-k, k))
else East := [] end if;
RETURN([op(West), op(East)])
end if;
end proc;
ZL :=[S, {S = Set(Cycle(Z), 3 <= card)}, unlabelled]: seq(combstruct[count](ZL, size=n), n=1..41); # Zerinvary Lajos, Mar 25 2008
B:=[S, {S = Set(Sequence(Z, 1 <= card), card >=3)}, unlabelled]: seq(combstruct[count](B, size=n), n=1..41); # Zerinvary Lajos, Mar 21 2009
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MATHEMATICA
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Length /@ Table[Select[Partitions[n], Length[#] > 2 &], {n, 20}] (* Eric W. Weisstein, May 16 2007 *)
Table[Count[Length /@ Partitions[n], _?(# > 2 &)], {n, 20}] (* Eric W. Weisstein, May 16 2017 *)
Length /@ Table[IntegerPartitions[n, {3, n}], {n, 20}] (* Eric W. Weisstein, May 16 2017 *)
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PROG
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(PARI) a(n) = numbpart(n) - (n+2)\2; /* Joerg Arndt, Apr 03 2013 */
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CROSSREFS
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A008284 counts partitions by sum and length.
A027187 counts partitions of even length.
A309356 ranks simple covering graphs.
The following count vertex-degree partitions and give their Heinz numbers:
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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