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A111143
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Number of different ways of drawing chords in a circle of numbers from 1 to n such that the sums of the numbers on the two sides of the chord are equal.
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1
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1, 0, 0, 1, 1, 0, 0, 2, 1, 0, 0, 1, 2, 2, 0, 1, 1, 0, 0, 2, 1, 0, 1, 1, 2, 0, 0, 1, 1, 0, 2, 2, 1, 0, 0, 1, 2, 1, 0, 1, 1, 0, 0, 2, 1, 0, 1, 1, 3, 0, 0, 1, 2, 1, 1, 2, 1, 0, 0, 1, 2, 4, 3, 2, 1, 0, 0, 3, 1, 0, 0, 1, 2, 0, 0, 2, 1, 0, 3, 2, 1, 0, 0, 1, 2, 1, 0, 1, 4, 0, 0, 2, 1, 4, 0, 1, 3, 0
(list; graph; refs; listen; history; internal format)
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OFFSET
| 2,8
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LINKS
| Alois P. Heinz, Table of n, a(n) for n = 2..10000
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EXAMPLE
| a(5) = 1 because in a circle with the numbers from 1 to 5 we can put a chord from 1 and 4 and the sum of both sides is the same.
a(9) = 2 as in a circle with the numbers from 1 to 9 we can put a chord in two ways, one from 2 to 7 and another from 6 to 9.
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MAPLE
| a:= proc(n) local c, i, j, u, v;
c, i, j, u, v:= 0, 1, 2, 0, n*(n+1)/2-3;
while j<=n do
c:= c + `if` (u=v, 1, 0);
if u>v then u, v:= u-i-1, v+i; i:=i+1
else u, v:= u+j, v-j-1; j:=j+1
fi;
od; c
end:
seq (a(n), n=2..100); # Alois P. Heinz, Sep 12 2011
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CROSSREFS
| Sequence in context: A014674 A015339 A137867 * A004197 A048571 A025880
Adjacent sequences: A111140 A111141 A111142 * A111144 A111145 A111146
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KEYWORD
| easy,nonn
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AUTHOR
| Joao B. Oliveira (oliveira(AT)inf.pucrs.br), Oct 18 2005
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