function xdot=center(t,x)
% Case: Node-Center

xdot(1)=-x(1)-x(2);
xdot(2)=4.*x(1)+x(2);
xdot=xdot';


% "Complex and Chaotic Nonlinear Dynamics. 
% Advances in Economics and Finance, 
% Mathematics and Statistics" 
% T.Vialar, Springer 2009
% Copyright(c).