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A171842
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Binomial transform of 1,0,1,0,2,0,4,0,8,0,16,...
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2
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1, 1, 2, 4, 9, 21, 50, 120, 289, 697, 1682, 4060, 9801, 23661, 57122, 137904, 332929, 803761, 1940450, 4684660, 11309769, 27304197, 65918162, 159140520, 384199201, 927538921, 2239277042, 5406093004, 13051463049, 31509019101, 76069501250, 183648021600, 443365544449, 1070379110497
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graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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0,3
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COMMENTS
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Number of nonisomorphic n-element interval orders with no 3-element antichain. - Richard Stanley, Nov 21 2011
a(n) is the top left entry of the n-th power of the 3 X 3 matrix [1, 1, 0; 1, 1, 1; 0, 1, 1] or of the 3 X 3 matrix [1, 0, 1; 0, 1, 1; 1, 1, 1]. - R. J. Mathar, Feb 03 2014
a(n) is the number of Motzkin n-paths of height <= 2. - Alois P. Heinz, Nov 24 2023
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LINKS
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FORMULA
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a(n) = 3*a(n-1)-a(n-2)-a(n-3). G.f.: (1-2*x)/((1-x)*(1-2*x-x^2)). - Colin Barker, Apr 01 2012
a(n) = (2+(1-sqrt(2))^n+(1+sqrt(2))^n)/4. - Colin Barker, Mar 16 2016
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MAPLE
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read("transforms") :
L := [1, seq(2^i, i=0..30)] ;
AERATE(L, 1) ;
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MATHEMATICA
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PROG
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(PARI) Vec((1-2*x)/((1-x)*(1-2*x-x^2)) + O(x^50)) \\ Colin Barker, Mar 16 2016
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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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