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 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 (list; graph; refs; listen; history; text; internal format)
 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)/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 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

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Last modified May 21 10:48 EDT 2019. Contains 323443 sequences. (Running on oeis4.)