I want to prove that a system is stable using a delta-epsilon proof.
My criteria for stability is: for all $t>0$, $\epsilon>0$ and $\delta>0$, $||x(0)||<\delta$ and $||x(t)||<\epsilon$. Where I want to find $\delta$ as a function of $\epsilon$ to satisfy the criteria.
My system is in state space form (basically two first order ODE's):
$\begin{bmatrix}\dot{x}_1\\\dot{x}_2\end{bmatrix}=$ $\begin{bmatrix}0&1\\-\zeta^2&-2\zeta\end{bmatrix}$ $\begin{bmatrix}x_1\\x_2\end{bmatrix}$
But I have rewritten the system as one 2nd order ODE:
$\ddot{x} + 2\zeta\dot{x} + \zeta^2x=0$
I found the solution to be $x(t)=e^{-\zeta t}(C_1 + C_2t)$
We can see that $x(t=0)=C_1$ and $x(t=\infty)=0$
So my question is where to go from here? I know my system converges to $0$, but I'm not sure how to find the correct $\delta(\epsilon)$?
Do I start with this ?
$$0<\delta$$ and $$e^{-\zeta t}(C_1 + C_2t)<\epsilon$$