%I #5 Mar 31 2012 12:36:12
%S 9,45,117,565,1953,9153,37269,173409,766869,3589357,16545861,78195505,
%T 368573829,1757922765,8394885777,40340645213,194259462205,
%U 938930726781,4547344031581,22077243778089,107366485176765,523060579628697
%N Number of arrangements of n+1 numbers x(i) in -2..2 with the sum of x(i)*x(i+1) equal to zero
%C Column 2 of A188358
%H R. H. Hardin, <a href="/A188351/b188351.txt">Table of n, a(n) for n = 1..200</a>
%e Some solutions for n=4
%e ..2...-1....1...-1...-2...-1....0....0....0...-1...-1....2....0...-2....1...-1
%e ..0...-2....2....2....1....1...-1....0...-2...-2...-1....0....0....2....2....1
%e ..0....1...-2...-1....0....1....0....0....0...-2...-2...-2....2....1....0....1
%e ..2....0...-2...-2...-2....2....1...-1...-2....2....1....1....0....1...-1....0
%e ..0...-1....1...-1...-1...-1....0....0....0...-1...-1....2....0....1....2...-1
%K nonn
%O 1,1
%A _R. H. Hardin_ Mar 28 2011