# FAQ

### How can I save a solution every n steps, or at specific parameter values?

You can use the callback `finaliseSolution`

in the function call `continuation`

. For example, you can use something like this to save all steps

```
function mySave(u, tau, step, contResult, personaldata)
push!(personaldata, u)
end
```

and pass it like `continuation(prob, alg, opts; finaliseSolution = (z, tau, step, contResult; k...) -> mySave(z, tau, step, contResult, myData))`

### The Fold / Hopf Continuation does not work, why?

This requires some precise computations. Have you tried passing the expression of the Jacobian instead of relying on finite differences.

### What is the parameter `theta`

about in `ContinuationPar`

?

See the description of continuation on the page Library.

### How can I change the preconditioner during computations?

The easiest way to achieve this is by using the callbacks provided by `newton`

and `continuation`

. See the documentation about these two methods. See also the example 2d Ginzburg-Landau equation (finite differences, codim 2, Hopf aBS)

### How can I implement my own bifurcation detection method?

You can use the callback `finaliseSolution`

but the best way is probably to use the Iterator Interface to inject your code anywhere in the continuation procedure.

### How do I dissociate the computation of eigenvalues from the jacobian that I passed?

Sometimes, for example when implementing boundary conditions, you pass a jacobian `J`

but the eigenvalues, and the bifurcation points are not simply related to `J`

. One way to bypass this issue is to define a new eigensolver `<: AbstractEigenSolver`

and pass it to the `NewtonPar`

field `eigsolver`

. This is done for example in `example/SH2d-fronts-cuda.jl`

.

### How can I print the eigenvalues during `continuation`

?

You can print the eigenvalues using the following callback:

```
finaliseSolution = (z, tau, step, contResult; k...) -> begin
BK.haseigenvalues(contResult) && Base.display(contResult.eig[end].eigenvals)
return true
end,
```

### How can I reject a Newton Step?

You can reject a newton step by passing to `continuation`

the argument `callbackN`

```
function mycallback(x, f, J, res, iteration, itlinear, options; kwargs...)
# stop Newton algo if residual too large
if res > 1e2
@warn "Reject Newton step!!"
return false
end
return true
end
```

### How do I stop `continuation`

?

Using the argument `finaliseSolution`

in `continuation`

. Simply make this function `finaliseSolution`

return false.

### How do I compute both sides of a branch?

Instead of using two calls to `continuation`

, you can pass the keyword `bothside=true`

to `continuation`

### How do I compute period orbits for non-autonomous problems

The package does not yet allow to compute periodic orbits solutions of non-autonomous Cauchy problems like

\[\frac{du}{dt} = F(u, par, t).\]

On certains cases, one can still go away with the following trick. Say one is interested (dummy example!) to study

\[\dot u = cos(u) + cos(\omega \cdot t).\]

Then one can use the following autonomous vector field

```
function vectorField(U, par)
u, x, y = U
out = similar(U)
out[1] = cos(u) + x
x2 = x^2+y^2
out[2] = x + par.ω * y - x * x2
out[3] = y - par.ω * x - y * x2
out
end
```