# Optimization problems in non-linear and linear differential equations

I am looking for references on the implementation of computational optimization methods within the context of differential equations.

As an example, let's say I have several coupled non-linear ordinary differential equations describing a biogeochemical system where some living organism $$X$$ eats chemical compounds $$c_1$$, $$c_2$$ and $$c_3$$ that contain some energy for living and reproduction ($$\Omega_1$$, $$\Omega_2$$ and $$\Omega_3$$). This energy is variable in time and depends on the concentrations of $$c_1$$, $$c_2$$ and $$c_3$$:

\begin{aligned} \dot c_1 &= - X \cdot c_1 \cdot \alpha\\ \dot c_2 &= - X \cdot c_2 \cdot \beta \\ \dot c_3 &= - X \cdot c_3 \cdot \gamma\\ \dot X &= \Omega_1 (c_1, c_2 ,c_3)\cdot X \cdot c_1 \cdot \alpha + \Omega_2(c_1, c_2 ,c_3)\cdot X \cdot c_2 \cdot \beta + \Omega_3(c_1, c_2 ,c_3)\cdot X \cdot c_3 \cdot \gamma \end{aligned}

where

$$\alpha + \beta + \gamma = 1$$

At every timestep, an optimization algorithm finds the maximum

$$\max \bigg( \Omega_1 (c_1)\cdot X \cdot c_1 \cdot \alpha + \Omega_2(c_2)\cdot X \cdot c_2 \cdot \beta + \Omega_3(c_3)\cdot X \cdot c_3 \cdot \gamma \bigg)$$

i.e., to find the best value of $$\alpha, \beta, \gamma$$ to ensure maximum growth of $$X$$.

The problem I am facing is that the solution is not smooth, i.e., on some time step living organism picks best energy-yielding product $$c_1$$, but the next time step it picks $$c_2$$, so it produces non-smooth solutions for $$\alpha, \beta, \gamma$$ and it kind of jumps back and forth between $$c_1$$, $$c_2$$ and $$c_3$$ in time, which is not very realistic. So I need some kind of optimization algorithms that has smooth switching functions, e.g., sigmoid functions, but for multiple variables $$\alpha, \beta$$, and $$\gamma$$.

This is a toy example. In the real example, I am solving PDE of nonlinear equations and have more than $$30$$ variables of $$\alpha$$, $$\beta$$, and $$\gamma$$.

I found some information about receding horizon algorithm for the system that is very close to the system I described above (link). However, the description is very general and no information on the implementation.

• What can you tell about functions $\Omega_i$? – Rodrigo de Azevedo Apr 27 at 15:27
• Are $\alpha, \beta, \gamma$ parameters or time-varying control inputs? – Rodrigo de Azevedo Apr 27 at 15:30
• $\Omega$ is the function of $c$. Something like $\Omega = A + RT ln (c_1^m)/(c_2^n)$ – Igor Markelov Apr 28 at 12:23
• In the question, there are functions $\Omega_1, \Omega_2, \Omega_3$, each depending on a single input, namely, $c_1, c_2, c_3$, respectively. Yet, your function $\Omega$ (where is it used?) contains two inputs. – Rodrigo de Azevedo Apr 28 at 12:33
• For example, $\Omega_1$ will have $c_1$ for sure, but It may also have other $c_i$. In the real example, it is Gibbs Free Energy, $G = G_0 + RT ln A^mB^n/C^jD^k$, where $A$, $B$, $C$, $D$ are the concentrations in the chemical reaction $A$ + $B$ -> $C$ + $D$ and $G_0$, R and T are constants. In other words $\Omega = f(c_1, c_n)$ – Igor Markelov Apr 28 at 12:42

Hint: I will assume that $$\alpha$$, $$\beta$$ and $$\gamma$$ are constants. You can simplify your problem by eliminating $$c_2$$ and $$c_3$$ by dividing the first and second and the first and the third differential equation.
$$\dfrac{dc_1}{dc_2}=\dfrac{\alpha}{\beta}\dfrac{c_1}{c_2}\implies \dfrac{dc_1}{c_1}=\dfrac{\alpha}{\beta}\dfrac{dc_2}{c_2}$$ $$\implies \ln c_1 -\ln c_1(t=0)=\dfrac{\alpha}{\beta}[\ln c_2 -\ln c_2(t=0)]$$ Now, solve for $$c_2$$ to obtain
$$c_2=c_2(0)\left[\frac{c_1}{c_1(0)} \right]^{\beta/\alpha}.$$