function [ydot] = aren(t, y)

mu = 0.012277471;
r1 = sqrt((y(1)+mu)^2 + y(2)^2);
r2 = sqrt((y(1)-1+mu)^2 + y(2)^2);

ydot(1) = y(3);
ydot(2) = y(4);
ydot(3) = y(1);
ydot(4) = y(2);