# Asymptotic expansion on 3 nonlinear ordinary differential equations

The 3 nonlinear differential equations are as follows $$\epsilon \frac{dc}{dt}=\alpha I + \ c (-K_F - K_D-K_N s-K_P(1-q)), \nonumber$$
$$\frac{ds}{dt}= \lambda_b P_C \ \epsilon \ c (1-s)- \lambda_r (1-q) \ s, \nonumber$$ $$\frac{dq}{dt}= K_P (1-q) \frac{P_C}{P_Q} \ \ c - \gamma \ q, \nonumber$$ I want to use asymptotic expansion on $c, s$ and $q$. And values of parameters are:

$K_F = 6.7 \times 10^{-2},$

$K_N = 6.03 \times 10^{-1}$

$K_P = 2.92 \times 10^{-2}$,

$K_D = 4.94 \times 10^{-2}$,

$\lambda_b= 0.0087$,

$I=1200$

$P_C = 3 \times 10^{11}$

$P_Q = 2.304 \times 10^{9}$

$\gamma=2.74$

$\lambda_{b}=0.0087$

$\lambda_{r}= 835$

$\alpha=1.14437 \times 10^{-3}$

For initial conditions:

$$c_0(0)= c(0) = 0.25 \nonumber$$ $$s_0(0)= cs(0) = 0.02 \nonumber \nonumber$$ $$q_0(0)=q(0) = 0.98 \nonumber \nonumber$$ and $$c_i(0)= 0, \ i>0\nonumber$$ $$s_i(0)= 0, \ i>0 \nonumber \nonumber$$ $$q_i(0)=0, i>0. \nonumber \nonumber$$

=> i started with the expansions : $$c= c_0+ \epsilon c_1 + \epsilon^2 c_2+......... \nonumber$$ $$s= s_0+ \epsilon s_1 + \epsilon^2 s_2+......... \nonumber$$ $$q= q_0+ \epsilon q_1 + \epsilon^2 q_2+......... \nonumber$$ we are only interseted in up to fisrt power of $\epsilon$. so, we should get total 6 approximate differential equations to get answer for $\dfrac{dc_0}{dt}, \dfrac{ds_0}{dt}, \dfrac{dq_0}{dt}, \dfrac{dc_1}{dt}, \dfrac{ds_1}{dt}$ and $\dfrac{dq_1}{dt}$

but i think $\dfrac{dc_1}{dt}$ will disappear while expanding and equating the up to first power of $\epsilon$, do i need to go further up to $\epsilon{^2}$ because $\dfrac{dc_1}{dt}$ is very important to find and we need 6 approximate differetial equations in total. what can i do? please some one help me.

• What is the variable $I$ in the first equation? Jul 25 '14 at 21:26
• variable $I=1200$ Jul 25 '14 at 21:41
• I've added the tags differential-equations and perturbation-theory and removed the tag math-software since it didn't seem to be relevant. Please re-add it though if you feel it should be there. Jul 28 '14 at 3:09

This is an example of a singular perturbation problem and so the type of perturbative expansion you are attempting is not likely to give you anything useful. I would recommend reading about multi-scale analysis, which can be found in most graduate books on nonlinear differential equations (Glenddings book has an introduction).
For a detailed exposition i would refer you to Kervokian and Coles book.

The system you have presented has $3$ time scales; fast time $\epsilon^{-1}t$, 'slow' time $t$, and 'really slow' time $\epsilon t$. On the fast time scale, $c$ will rapidly approach the slow manifold, ie the fixed point at $$\dot{c} =0 \implies c(q,s) = \frac{I\alpha}{(K_F+K_D+K_Ns+K_P(1−q))}.$$

You can then plug this into the equations for $\dot{s}$ and $\dot{q}$ to produce a two dimensional nonlinear equation for the motion on this slow manifold. Note that in this reduced system, there is still $\epsilon$ floating around. Since it is not a coefficient of a derivative term, the perturbative expansion attempted in the original post will work.