## 13.2 Example: simple harmonic oscillator

As an example of a system of ODEs, consider a harmonic oscillator. In a harmonic oscillator a particle disturbed from equilibrium is pulled back towards its equilibrium position by a force proportional to its displacement from equilibrium. The system here additionally has a friction force proportional to particle speed which points in the opposite direction of the particle velocity. The system state will be a pair $$y = (y_1, y_2)$$ representing position and speed. The change in the system with respect to time is given by the following differential equations.25

$\frac{d}{dt} y_1 = y_2 \\ \frac{d}{dt} y_2 = -y_1 - \theta y_2$

The state equations implicitly defines the state at future times as a function of an initial state and the system parameters.

### References

Ahnert, Karsten, and Mario Mulansky. 2011. “Odeint—Solving Ordinary Differential Equations in C++.” arXiv 1110.3397.

1. This example is drawn from the documentation for the Boost Numeric Odeint library , which Stan uses to implement the rk45 and ckrk solver.↩︎