

A111143


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.


1



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
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OFFSET

2,8


LINKS

Alois P. Heinz, Table of n, a(n) for n = 2..10000


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.


MAPLE

a:= proc(n) local c, i, j, u, v;
c, i, j, u, v:= 0, 1, 2, 0, n*(n+1)/23;
while j<=n do
c:= c + `if` (u=v, 1, 0);
if u>v then u, v:= ui1, v+i; i:=i+1
else u, v:= u+j, vj1; j:=j+1
fi;
od; c
end:
seq (a(n), n=2..100); # Alois P. Heinz, Sep 12 2011


CROSSREFS

Sequence in context: A015339 A137867 A324734 * A004197 A261684 A048571
Adjacent sequences: A111140 A111141 A111142 * A111144 A111145 A111146


KEYWORD

easy,nonn


AUTHOR

Joao B. Oliveira (oliveira(AT)inf.pucrs.br), Oct 18 2005


STATUS

approved



