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A008484
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Number of partitions of n into parts >= 4.
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26
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1, 0, 0, 0, 1, 1, 1, 1, 2, 2, 3, 3, 5, 5, 7, 8, 11, 12, 16, 18, 24, 27, 34, 39, 50, 57, 70, 81, 100, 115, 140, 161, 195, 225, 269, 311, 371, 427, 505, 583, 688, 791, 928, 1067, 1248, 1434, 1668, 1914, 2223, 2546
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OFFSET
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0,9
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COMMENTS
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a(n) is also the number of not necessarily connected 2-regular graphs on n-vertices with girth at least 4 (all such graphs are simple). The integer i corresponds to the i-cycle; addition of integers corresponds to disconnected union of cycles. - Jason Kimberley, Jan 2011 and Feb 2012
By removing a single part of size 4, an A026797 partition of n becomes an A008484 partition of n - 4. Hence this sequence is essentially the same as A026797. - Jason Kimberley, Feb 2012
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LINKS
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Table of n, a(n) for n=0..49.
Jason Kimberley, Not necessarily connected k-regular graphs with girth at least 4
Jason Kimberley, Index of sequences counting not necessarily connected k-regular simple graphs with girth at least g
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FORMULA
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G.f.: Product 1/(1-x^m); m=4..inf.
Euler transformation of A185114. - Jason Kimberley, Jan 30 2011
Given by p(n)-p(n-1)-p(n-2)+p(n-4)+p(n-5)-p(n-6) where p(n)=A000041(n). Generally, 1/product(i=K, oo, 1-x^i) is given by p({A}), where {A} is defined over the coefficients of product(i=1, K-1, 1-x^i). In this case K=4, so (1-x)(1-x^2)(1-x^3)=1-x-x^2+x^4+x^5-x^6, defining {A} as above. G.f.: 1 + sum(i=1, oo, x^4i/product(j=1, i, 1-x^j)) - Jon Perry, Jul 04 2004
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MAPLE
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series(1/product((1-x^i), i=4..50), x, 51);
ZL := [ B, {B=Set(Set(Z, card>=4))}, unlabeled ]: seq(combstruct[count](ZL, size=n), n=0..49); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Mar 13 2007
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MATHEMATICA
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f[1, 1] = 1; f[n_, k_] := f[n, k] = If[n < 0, 0, If[k > n, 0, If[k == n, 1, f[n, k + 1] + f[n - k, k]]]]; Table[ f[n, 4], {n, 49}]
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CROSSREFS
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2-regular graphs with girth at least 4: A185114 (connected), A185224 (disconnected), this sequence (not necessarily connected).
Not necessarily connected 2-regular graphs with girth at least g [partitions into parts >= g]: A026807 (triangle); chosen g: A000041 (g=1 -- multigraphs with loops allowed), A002865 (g=2 -- multigraphs with loops forbidden), A008483 (g=3), this sequence (g=4), A185325(g=5), A185326 (g=6), A185327 (g=7), A185328 (g=8), A185329 (g=9).
Not necessarily connected 2-regular graphs with girth exactly g [partitions with smallest part g]: A026794 (triangle); chosen g: A002865 (g=2), A026796 (g=3), A026797 (g=4), A026798 (g=5), A026799 (g=6), A026800(g=7), A026801 (g=8), A026802 (g=9), A026803 (g=10).
Not necessarily connected k-regular simple graphs with girth at least 4: A185314 (any k), A185304 (triangle); specified degree k: this sequence (k=2), A185334 (k=3), A185344 (k=4), A185354 (k=5), A185364 (k=6).
Sequence in context: A032230 A126793 A069910 * A026797 A027189 A140829
Adjacent sequences: A008481 A008482 A008483 * A008485 A008486 A008487
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KEYWORD
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nonn,easy
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AUTHOR
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T. Forbes (anthony.d.forbes(AT)googlemail.com)
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STATUS
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approved
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