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A319981
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a(n) is the number of integer partitions of n with largest part <= 3 for which the index of the seaweed algebra formed by the integer partition paired with its weight is 0.
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5
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1, 1, 1, 2, 2, 1, 2, 1, 2, 1, 2, 1, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2
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OFFSET
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1,4
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COMMENTS
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The index of a Lie algebra, g, is an invariant of the Lie algebra defined by min(dim(Ker(B_f)) where the min is taken over all linear functionals f on g and B_f denotes the bilinear form f([_,_]) were [,] denotes the bracket multiplication on g.
For seaweed subalgebras of sl(n), which are Lie subalgebras of sl(n) whose matrix representations are parametrized by an ordered pair of compositions of n, the index can be determined from a corresponding graph called a meander.
a(n) is periodic with period 2 for n>12.
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LINKS
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FORMULA
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For n>12: a(n)=2 if n is odd, a(n)=0 if n is even.
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MAPLE
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1, 1, 1, 2, 2, 1, 2, 1, 2, 1, 2, 1, 2 seq(op([0, 2]), n=1..30); # Muniru A Asiru, Dec 07 2018
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MATHEMATICA
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Join[{1, 1, 1, 2, 2, 1, 2, 1, 2, 1, 2, 1}, LinearRecurrence[{0, 1}, {2, 0}, 100]] (* Jean-François Alcover, Dec 07 2018 *)
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PROG
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(GAP) a:=[1, 1, 1, 2, 2, 1, 2, 1, 2, 1, 2, 1, 2];; Concatenation(a, Flat(List([1..30], n->[0, 2]))); # Muniru A Asiru, Dec 07 2018
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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