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A000297
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a(n) = (n+1)*(n+3)*(n+8)/6.
(Formerly M3434 N1393)
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10
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0, 4, 12, 25, 44, 70, 104, 147, 200, 264, 340, 429, 532, 650, 784, 935, 1104, 1292, 1500, 1729, 1980, 2254, 2552, 2875, 3224, 3600, 4004, 4437, 4900, 5394, 5920, 6479, 7072, 7700, 8364, 9065, 9804, 10582, 11400, 12259, 13160, 14104, 15092, 16125, 17204
(list;
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listen;
history;
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OFFSET
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-1,2
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COMMENTS
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If Y and Z are 2-blocks of an n-set X then, for n>=4, a(n-5) is the number of (n-3)-subsets of X intersecting both Y and Z. - Milan Janjic, Nov 09 2007
a(n) is the number of triangles in the Turan graph T(n, n-2) for n>3. - Robert H Cowen, Feb 25 2018
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REFERENCES
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J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 150.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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G.f.: (2-x)^2 / (1-x)^4.
G.f.: 2*x*W(0), where W(k) = 1 + 1/( 1 - x*(k+2)*(k+4)*(k+9)/(x*(k+2)*(k+4)*(k+9) + (k+1)*(k+3)*(k+8)/W(k+1) )) ); (continued fraction). - Sergei N. Gladkovskii, Aug 24 2013
With offset 3, for n>3, a(n) = 4 binomial(n-2,2) + binomial(n-3,3), comprising the fourth column of A267633. - Tom Copeland, Jan 25 2016
a(n) = a(n-1) + (n^2 + 7n + 8)/2.
(End)
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MAPLE
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MATHEMATICA
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Table[(n + 1)*(n + 3)*(n + 8)/6, {n, -1, 100}]
CoefficientList[Series[x (2 - x)^2 / (1 - x)^4, {x, 0, 50}], x] (* Vincenzo Librandi, Oct 31 2018 *)
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PROG
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(PARI) a(n) = (n+1)*(n+3)*(n+8)/6; \\ Altug Alkan, Jan 10 2015
(GAP) List([-1..45], n->(n+1)*(n+3)*(n+8)/6); # Muniru A Asiru, Mar 11 2018
(Python)
def A000297_gen(): # generator of terms
a, b, c = 0, 4, 4
while True:
yield a
a, b, c = a+b, b+c, c+1
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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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STATUS
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approved
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