Petar Mlinarić

There is still a lot of repetition, but a lot of differences, in the subclasses of `GenericIRKAReductor`.

Topics to cover: - `IRKAReductor` - `TSIAReductor` - maybe also structured reductors

Links with `!` should be improved (see 31fe516dde04c92e398e610971c97f555f622f90), perhaps by marking the class/method as removed/moved/renamed, or by making the links work for attributes and some private methods.

Some classes document parameters (e.g., [`NumpyVectorSpace`](https://github.com/pymor/pymor/blob/4eaeb774dc295e6685a3c509a8cb9f4788e0b7d3/src/pymor/vectorarrays/numpy.py#L208)), some attributes (e.g., [`VectorArray`](https://github.com/pymor/pymor/blob/4eaeb774dc295e6685a3c509a8cb9f4788e0b7d3/src/pymor/vectorarrays/interface.py#L14)), some both (e.g., [`LTIModel`](https://github.com/pymor/pymor/blob/4eaeb774dc295e6685a3c509a8cb9f4788e0b7d3/src/pymor/models/iosys.py#L59)), and some neither (e.g., [`ListVectorSpace`](https://github.com/pymor/pymor/blob/4eaeb774dc295e6685a3c509a8cb9f4788e0b7d3/src/pymor/vectorarrays/list.py#L524)). We should decide on a policy.

Add option to specify frequency units

4

Right now, all of the methods of [`TransferFunction`](https://github.com/pymor/pymor/blob/4866fa087df441be1ffc8ebb09bd6ee34bed6cf1/src/pymor/models/transfer_function.py) that accept `w` assume that it is given in rad/s. To unify continuous and discrete-time systems, it would be good to also...

It would be interesting to add support for parametric eigenvalue problems $A(\mu) x(\mu) = \lambda(\mu) x(\mu)$, where the goal is to approximate a few eigenvalues $\lambda(\mu)$ and/or the corresponding eigenvectors...

Recycled Krylov subspace methods

1

To be clear, I know barely anything about these methods. I only heard that they can significantly improve performance of IRKA (a Krylov-based MOR method), but it is not my...

Currently [PAAA](https://github.com/pymor/pymor/blob/aff70232b38996c94d009d6d9b6f6feef66de0dc/src/pymor/reductors/aaa.py#L69-L138), [Loewner](https://github.com/pymor/pymor/blob/aff70232b38996c94d009d6d9b6f6feef66de0dc/src/pymor/reductors/loewner.py#L49-L106), and [VF](https://github.com/pymor/pymor/blob/aff70232b38996c94d009d6d9b6f6feef66de0dc/src/pymor/reductors/h2.py#L855-L891) contain potentially expensive computation. It would be good to perform it in the first call to the `reduce` method.