Hi all,

Being new to Julia, I started playing around a bit with differential equations.

I started with a simple system, du = A * u where A is an n*n array, and u a vector of length n.

when I set up the system as

`f(u,p,t) = A*u`

and provide initial conditions, it works as intended.

However, when I instead set it up as

```
function fun!(du,u,p,t)
du = A*u
end
```

the solution maintains stuck at the initial value.

It does work when I write out equations explicitly (du[1] = …, du[2] = …, etc.), but this is not very practical. Also, the size & type of du are exactly the same when I do the matrix multiplication as when I write the equations explicitly. So, I’m a bit puzzled why the matrix multiplication doesn’t work.

Would appreciate some insights!