English: Time series and phase portrait of the solution ${\displaystyle x(t)}$ of the Duffing equation, ${\displaystyle {\ddot {x}}+\delta {\dot {x}}+\alpha x+\beta x^{3}=\gamma \cos(\omega t)\,}$ for ${\displaystyle \gamma =0.29,}$ showing a period-4 oscillation due to period doubling. The other parameters are: ${\displaystyle \alpha =-1,}$ ${\displaystyle \beta =+1,}$ ${\displaystyle \delta =0.3}$ and ${\displaystyle \omega =1.2.}$ The initial conditions are ${\displaystyle x(0)=1}$ and ${\displaystyle {\dot {x}}(0)=0.}$
The time series shows ${\displaystyle x}$ as a function of ${\displaystyle t/T,}$ with ${\displaystyle T=2\pi /\omega }$ the period. The phase portrait depicts the time series plotted in the ${\displaystyle x}$–${\displaystyle {\dot {x}}}$ phase plane. The red dots depict the moments when ${\displaystyle t}$ is an integer multiple of the period ${\displaystyle T.}$