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A027336
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Number of partitions of n that do not contain 2 as a part.
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39
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1, 1, 1, 2, 3, 4, 6, 8, 11, 15, 20, 26, 35, 45, 58, 75, 96, 121, 154, 193, 242, 302, 375, 463, 573, 703, 861, 1052, 1282, 1555, 1886, 2277, 2745, 3301, 3961, 4740, 5667, 6754, 8038, 9548, 11323, 13398, 15836, 18678, 22001, 25873, 30383, 35620, 41715, 48771
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OFFSET
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0,4
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COMMENTS
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Pairwise sums of sequence A002865 (partitions in which the least part is at least 2).
Also number of partitions of n into parts with at most one 1. - Reinhard Zumkeller, Oct 25 2004
Also number of partitions of n into parts with at least half of the parts having size 1; equivalently (by duality) number of partitions of n where the large part is at least twice as big as the second largest part. - Franklin T. Adams-Watters, Jun 08 2005
Also number of 2-regular not necessarily connected graphs with loops allowed but no multiple edges. - Jason Kimberley, Jan 05 2011
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LINKS
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FORMULA
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G.f.: (1 - x^2)*Product_{m>=1} 1/(1 - x^m).
a(n) = p(n) - p(n-2) for n >= 2, where p(n) are the partition numbers (A000041); follows at once from the g.f. - Emeric Deutsch, Feb 18 2006
a(n) ~ exp(sqrt(2*n/3)*Pi)*Pi / (6*sqrt(2)*n^(3/2)) * (1 - (3*sqrt(3/2)/Pi + 25*Pi/(24*sqrt(6)))/sqrt(n) + (25/8 + 9/(2*Pi^2) + 817*Pi^2/6912)/n). - Vaclav Kotesovec, Nov 04 2016
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MAPLE
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with(combinat): a:=proc(n) if n=0 then 1 elif n=1 then 1 else numbpart(n)-numbpart(n-2) fi end: seq(a(n), n=0..49); # Emeric Deutsch, Feb 18 2006
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MATHEMATICA
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PROG
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(PARI) a(n)=if(n<0, 0, polcoeff((1-x^2)/eta(x+x*O(x^n)), n))
(Magma) A41 := func<n|n ge 0 select NumberOfPartitions(n) else 0>;
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CROSSREFS
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2-regular not necessarily connected graphs: A008483 (simple graphs), A000041 (multigraphs with loops allowed), A002865 (multigraphs with loops forbidden), A027336 (graphs with loops allowed but no multiple edges). - Jason Kimberley, Jan 05 2011
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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