Hello,

I have a problem with solving a Differential Equation system properly. The code is working, but the solution is wrong (which I know for certain because I am reproducing results of someone else).

The system (in easy terms) is given by:

(d/dt)A(t) = -B(t)*g(A(t), t)

(d/dt)B(t) = B(t)^2 * f(A(t),t, (d/dt)A(t) )

where f and g are complicated functions, which also include integrals, but this integrals are not over t or A/B.

So I read the Tutorial for the DifferentialEquations package and also implemented some other easy systems correctly. Now I try to implement the System above as follows:

```
using DifferentialEquations
function Eqs(du,u,p,t)
du[1]=u[2]*g(u[1],t)
du[2]=u[2]^2 * f(u[1],du[1],t)
end
u0=[10^-5,1,0,0]
tspan(10^4, 10^-4) #solving "backwards"
prob=ODEProblem(Eqs,u0,tspan,p)
sol=solve(prob,AutoTsit5(Rosenbrock23()))]
```

After plotting the solution I can see that this is not correct. Moreover I tried the following:

```
sol_array = convert(Array,sol)
t_array = convert(Array,sol.t)
```

And look at the values for the derivatives:

```
print(sol_array[3:4,:])
```

Which (in my understanding) gives me the results for (d/dt)A and (d/dt)B for the timesteps. But this array is completely filled with 0 which I find very strange, because my values for A and B are growing, so how can the derivative be 0?

So I have 2 different possibilities for this:

A) I do not understand the Differentialequations Package correctly and the implementation of my system is wrong.

B) The Equation is very Stiff and the solver fails.

According to this stackoverflow post:

the different solver of the package are sophisticated enough to solve all not-too-exotic problems. I already used a variety of different solvers described in the documentation, but my problem still remains, such that I think that A) is my problem.

Any ideas?

edit: initial conditions are u0=[10^-5,1] not u0=[10^-5,0] and small mistake in DEQ

edit2: p added in definition