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A176734
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a(n) = (n+7)*a(n-1) + (n-1)*a(n-2), a(-1)=0, a(0)=1.
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4
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1, 8, 73, 746, 8425, 104084, 1395217, 20157542, 312129649, 5155334720, 90449857081, 1679650774658, 32908313146393, 678322072223756, 14672571587601985, 332293083938376254, 7862829504396683617, 194024597448534426872, 4984283037788104293289, 133083801736564331309210
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OFFSET
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0,2
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COMMENTS
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a(n) enumerates the possibilities for distributing n beads, n>=1, labeled differently from 1 to n, over a set of (unordered) necklaces, excluding necklaces with exactly one bead, and k=8 indistinguishable, ordered, fixed cords, each allowed to have any number of beads. Beadless necklaces as well as beadless cords contribute a factor 1 in the counting, e.g., a(0):= 1*1 =1. See A000255 for the description of a fixed cord with beads. This produces for a(n) the exponential (aka binomial) convolution of the subfactorial sequence {A000166(n)} and the sequence {A049388(n) = (n+7)!/7!}. See the necklaces and cords problem comment in A000153. Therefore the recurrence with inputs holds. This comment derives from a family of recurrences found by Malin Sjodahl for a combinatorial problem for certain quark and gluon diagrams (Feb 27 2010).
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LINKS
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FORMULA
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E.g.f. (exp(-x)/(1-x))*(1/(1-x)^8) = exp(-x)/(1-x)^9, equivalent to the given recurrence.
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EXAMPLE
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Necklaces and 8 cords problem. For n=4 one considers the following weak 2 part compositions of 4: (4,0), (3,1), (2,2), and (0,4), where (1,3) does not appear because there are no necklaces with 1 bead. These compositions contribute respectively !4*1,binomial(4,3)*!3*c8(1), (binomial(4,2)*!2)*c8(2), and 1*c8(4) with the subfactorials !n:=A000166(n) (see the necklace comment there) and the c8(n):=A049388(n) numbers for the pure 8-cord problem (see the remark on the e.g.f. for the k cords problem in A000153; here for k=8: 1/(1-x)^8). This adds up as 9 + 4*2*8 + (6*1)*72 + 7920 = 8425 = a(4).
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MATHEMATICA
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nxt[{n_, a_, b_}]:={n+1, b, (n+8)b+n*a}; Transpose[NestList[nxt, {1, 1, 8}, 20]][[2]] (* Harvey P. Dale, Mar 19 2013 *)
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CROSSREFS
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Cf. A176733 (necklaces and k=7 cords).
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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