Good evening to all,

I recently solved the ODEs system for the brusselator:

```
A -> X (k1)
2X + Y -> 3X (k2)
B + X -> Y + D (k3)
X -> E (k4)
```

where A, B, etc are chemical species and k represents kinetics coefficients.

Because I wanted to get the limit cycle ([Y] as a function of [ X]), I chose:

```
k1 = k2 = k3 = k4 = 1
[X]0 = [Y]0 = 1
[A] = 1 and [B] = 3
```

Here [ ] denotes the concentration in various species. The objective was to solve d[ X]/dt and d[Y]/dt. It is not difficult to fix the value of [A] and [B] since there is a direct access for the ODEs to be modified:

```
d[X]/dt = [A] + [Y][X]^2 - [B][X] -[X]
d[Y]/dt = -Y][X]^2 + [B][X]
```

I then became interested in Catalyst (so I solved the thermal decomposition of ethane as well as a problem of 2 consecutive organic reactions).

The next problem is to solve the brusselator with Catalyst. I defined:

```
brusselator = @reaction_network begin
k1, A β X
k2, 2X + Y β 3X
k3, B + X β Y + D
k4, X β E
end k1 k2 k3 k4
```

I disabled rescaling of reaction rates. The latexification shows that the ODEs so obtained are correct.

There is no problem to write **p** and **tspan**.

However how to formulate **u0** since [A] and [B] are not initial concentrations? How to introduce βexternalβ modifications to the ODEs system since this latter is no longer apparent?

At first sight I thought that some kind of forcing could help me. However I donβt see how to proceed on the basis of examples given here.

Could someone please guide me?

Thank you in advance,

Thierry