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A012781
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Take every 5th term of Padovan sequence A000931, beginning with the second term.
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6
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0, 1, 4, 16, 65, 265, 1081, 4410, 17991, 73396, 299426, 1221537, 4983377, 20330163, 82938844, 338356945, 1380359512, 5631308624, 22973462017, 93722435101, 382349636061, 1559831901918, 6363483400447, 25960439030624
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OFFSET
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0,3
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COMMENTS
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Number of nonisomorphic graded posets with 0 and uniform hasse graph of rank n, with exactly 2 elements of each rank level above 0, for n > 0. (Uniform used in the sense of Retakh, Serconek and Wilson.) Here, we do not assume all maximal elements have maximal rank and thus use graded poset to mean: For every element x, all maximal chains among those with x as greatest element have the same finite length. - David Nacin, Feb 13 2012
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REFERENCES
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R. Stanley, Enumerative combinatorics, Vol. 1, Cambridge University Press, Cambridge, 1997, pp. 96-100.
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LINKS
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FORMULA
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a(n+3) = 5*a(n+2) - 4*a(n+1) + a(n).
G.f.: x*(1-x)/(1-5*x+4*x^2-x^3). - Colin Barker, Feb 03 2012
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MATHEMATICA
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LinearRecurrence[{5, -4, 1}, {0, 1, 4}, 25] (* Harvey P. Dale, Jan 10 2012 *)
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PROG
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(Magma) I:=[0, 1, 4 ]; [n le 3 select I[n] else 5*Self(n-1)-4*Self(n-2)+Self(n-3): n in [1..40]]; // Vincenzo Librandi, Feb 03 2012
(Python)
def a(n, adict={0:0, 1:1, 2:4}):
if n in adict:
return adict[n]
adict[n]=5*a(n-1) - 4*a(n-2) + a(n-3)
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CROSSREFS
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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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