Mickens , Ronald E. and Wilkerson , Dorian. (2009). Exact and Approximate Values of the Period for a "Truly Nonlinear" Oscillator: $\ddot{x} + x + x^{1/3} = 0$.
            <i>Advances in Applied Mathematics and Mechanics</i>. <i>1</i> (3).
            383-390.
         doi: