An Epilogue of “Analyze This!”

In “Analyze This!“, we examined system

\begin{cases}\frac{dx}{dt}=n k_2 y - n k_1 x^n \\ \frac{dy}{dt}=k_1 x^n-k_2 y\\x(0)=x_0, y(0)=y_0\;\end{cases}

qualitatively.

Now, let us seek its equilibrium (x_*, y_*) quantitatively.

In theory, one may first solve differential equation

\frac{dx}{dt} = -nk_1x^n-k_2x+c_0

for x(t), using a popular symbolic differential equation solver such as ‘ode2’. Then compute x_* as \lim\limits_{t \rightarrow \infty} x(t), followed by y_* = \frac{k_1}{k_2}x_*^n.

However,in practice, such attempt meets a deadend rather quickly (see Fig. 1).

Fig. 1

Bring in a more sophisticated solver is to no avail (see Fig. 2)

Fig. 2

An alternative is getting x_* directly from polynormial equation

-nk_1x^n-k_2x+c_0=0\quad\quad\quad(1)

We can solve (1) for x if n \le 4. For example, when n=3, -3k_1 x^3-k_2x+c_0=0 has three roots (see Fig. 3).

Fig. 3

First two roots are complex numbers. By Descartes’ rule of signs, the third root

\frac{(\sqrt{4k_2^3+81c_0^2k_1}+9c_0\sqrt{k_1})^{\frac{2}{3}}-2^{\frac{2}{3}}k_2}{3 \cdot 2^{\frac{1}{3}}\sqrt{k_1}(\sqrt{4k_2^3+81c_0^2k_1}+9c_0\sqrt{k_1})^{\frac{1}{3}}}

is the x_* of equilibrium (x_*, y_*) (see Exercise-1).


Exercise-1 Show the third root is real and positive.

Exercise-2 Obtain x_* from -4k_1x^4-k_2x+c_0=0.

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