mu != 0 /\ (mu^3 + mu^2 - mu)/(mu + 3) = 0
<=> mu != 0 /\ mu (mu^2 + mu - 1)/(mu + 3) = 0 /**/
<=> mu != 0 /\ (mu = 0 \/ mu = -1/2 + sqrt(1/4 +1) \/ mu = -1/2 - sqrt(1/4 +1)) /**/
<=> mu = -1/2 + sqrt(5)/2 \/ mu = -1/2 - sqrt(5)/2 /**/
<=> mu = (-1 + sqrt(5))/2 \/ mu = (-1 - sqrt(5))/2