I found in Goldstein's Classical Mechanics that the condition for closed orbits is given by $\frac{d^2 V\_{eff}}{dr^2}>0$.(bertrand's theorem). Can somebody explain to me, how this inequality is related to the boundness of the orbits? I do not see it. Furthermore, I was wondering, whether this property is the only property that an effective potential has to fulfill in order to produce bounded orbits`?