Hello!

I try to solve a nonlinear differential equation using a split-step method.

The right hand side of my equation has two terms:

According to the split-step method, I can split every integration step (t_n, t_n+dt) into two (t_n, t_n+dt/2) and (t_n+dt/2,t_n+dt), and then replace the solution of the whole equation by a consecutive solution of the following equations:

The first equation takes u(t_n) as the initial condition and calculates u(t_n+dt/2). In turn, the second equation takes u(t_n+dt/2) as the initial condition and calculates u(t_n+dt). To solve the first equation I use my own method, but for the second one I want to use DIfferentialEquations. However, in the split-step method I have to change the initial condition of the problem on each step. But redefining the whole problem on each step is too costly. Therefore, I search for a method where I can define the problem once and then update the initial condition on demand.

So far, I think to use the integrators interface. Something like this:

```
using OrdinaryDiffEq
function g(du, u, p, t)
@inbounds @. du = u
return nothing
end
# Define the problem:
tspan = (0., 10.)
u0 = ones(100)
prob = ODEProblem(g, u0, tspan)
integrator = OrdinaryDiffEq.init(prob, BS3(), dense=false)
# Main loop:
Nt = 100
dt = (tspan[2] - tspan[1]) / Nt
u = copy(u0) # solution which is updated at each step
for i=1:Nt
# First half-step of the split-step method:
@. u = u + 0.01 # dummy example
# Second half-step of the split-step method:
@. integrator.u = u # set the new initial condition?
# @. integrator.uprev = u # for some reason, this does not work
step!(integrator, dt/2, true)
@. u = integrator.u # update the solution
end
```

However, are there any canonical method to update the initial condition?

Or may be there is some in-place solve method, which can use the original initial condition array u0 and update it with the solution on each step?

Thank you.