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A082397
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Number of directed aggregates of height <= 2 with n cells.
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1
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1, 2, 5, 11, 26, 62, 153, 385, 988, 2573, 6786, 18084, 48621, 131718, 359193, 985185, 2715972, 7521567, 20915256, 58373586, 163462815, 459136809, 1293223230, 3651864606, 10336625731, 29321683082, 83344398533, 237344961291
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OFFSET
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1,2
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COMMENTS
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REFERENCES
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Fouad Ibn-Majdoub-Hassani. Combinatoire de polyominos et des tableaux décalés oscillants. Thèse de Doctorat. 1991. Laboratoire de Recherche en Informatique, Université Paris-Sud XI, France.
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LINKS
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FORMULA
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a(n) = Sum_{k=1..n}(-1)^(k+1)*binomial(n+1, k+1)*binomial(k, floor((k-1)/2)). E.g.f.: -exp(x)*int(-BesselI(1, 2*x)+BesselI(2, 2*x), x)-exp(x)*(-BesselI(1, 2*x)+BesselI(2, 2*x)). - Vladeta Jovovic, Sep 18 2003
Conjecture D-finite with recurrence +(n+2)*a(n) +(-3*n-2)*a(n-1) -n*a(n-2) +3*n*a(n-3)=0. - R. J. Mathar, Jun 27 2022
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MAPLE
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add( (-1)^(k+1)*binomial(n+1, k+1)*binomial(k, floor((k-1)/2)), k=1..n) ;
end proc:
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MATHEMATICA
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Table[Sum[(-1)^(i+1)*Binomial[k+1, i+1] Binomial[i, Floor[(i-1)/2]], {i, 1, k}], {k, 1, 20}] (* Rigoberto Florez, Dec 10 2022 *)
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PROG
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(Python)
import math
def Sum(k):
S= sum((-1)**(i+1)*math.comb(k, i+1)*math.comb(i, math.floor((i-1)/2)) for i in range(1, k))
return S
for i in range (2, 20): print(Sum(i))
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CROSSREFS
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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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