This package contains several *randomized preconditioners*, which use
randomized numerical linear algebra to construct approximate inverses of matrices.
These approximate inverses can dramatically speed up iterative linear system solvers.

Given a positive semidefinite matrix `A`

, the Nyström Sketch `Â ≈ A`

is constructed by

```
using RandomizedPreconditioners
Â = NystromSketch(A, k, r)
```

where `k`

and `r`

are parameters with `k ≤ r`

.

We can use `Â`

to construct a preconditioner `P ≈ A + μ*I`

for the system
`(A + μ*I)x = b`

, which is solved by conjugate gradients.

If you need `P`

(e.g., `IterativeSolvers.jl`

), use

`P = NystromPreconditioner(Â, μ)`

If you need `P⁻¹`

(e.g., `Krylov.jl`

), use

`Pinv = NystromPreconditionerInverse(Anys, μ)`

These preconditioners can be simply passed into the solvers, for example

```
using Krylov
x, stats = cg(A+μ*I, b; M=Pinv)
```

The package `LinearSolve.jl`

defines
a convenient common interface to access all the Krylov implementations, which
makes testing very easy.

```
using RandomizedPreconditioners, LinearSolve
Â = NystromSketch(A, k, r)
P = NystromPreconditioner(Â, μ)
prob = LinearProblem(A, b)
sol = solve(prob, IterativeSolversJL_CG(), Pl=P)
```

The sketching algorithms below use a rank `r`

sketching matrix and have complexity
`O(n²r)`

. The parameter `k ≤ r`

truncates the sketch, which can improve numerical
performance. Possible choices include `k = r - 10`

and `k = round(Int, 0.95*r)`

.
Sketches allow for faster (approximate) multiplication (`*`

and `mul!`

) and are
used to construct preconditioners.

```
using RandomizedPreconditioners
Â = NystromSketch(A, k, r)
```

```
using RandomizedPreconditioners
Â = EigenSketch(A, k, r)
```

The Randomized SVD uses the powered randomized rangefinder [2, Alg. 9] with
powering parameter `q`

. Small values of `q`

(e.g., `5`

) seem to perform
well. Note that the complexity increases to `O(n²rq)`

.

```
using RandomizedPreconditioners
Â = RandomizedSVD(A, k, r; q=10)
```

We implement two algorithms for a randomized estimate of the maximum eigenvalue for a PSD matrix: the power method and the Lanczos method.

```
using RandomizedPreconditioners
const RP = RandomizedPreconditioners
λmax_power = RP.eigmax_power(A)
λmax_lanczos = RP.eigmax_lanczos(A)
λmin_lanczos = RP.eigmin_lanczos(A)
```

The Lanczos method can estimate the maximum and minimum eigenvalue simultaneously:

`λmax, λmin = RP.eig_lanczos(A; eigtype=0)`

There are several choices for the random embedding used in the algorithms.
By default, this package uses Gaussian embeddings (and Gaussian test matrices),
but it also includes the `SSFT`

and the ability to add new test matrices by
implementing the `TestMatrix`

interface.

A `TestMatrix`

, `Ω`

, should implement matrix multiplication for itself and its
adjoint by implementing the `!mul`

method.
See Martinsson and Tropp [2] Section 9 for more.

- Test Matrices
- TestMatrix type
- Sparse maps
- Subsampled trigonometric transform
- DCT & Hartley option for SSRFT
- Tensor product maps

- Rangefinders
- Lanzcos randomized rangefinder
- Chebyshev randomized rangefinder
- Incremental rangefinder with updating
- Subsequent orthogonalization
- A posteriori error estimation
- Incremental rangefinder with powering
- Incremental rangefinder for sparse matrices

- Sketches & Factorizations
- Powering option / incorporating rangefinder into Nystrom sketch
- powerURV (w. re-orthonormalization)
- CPQR decomposition
- Improve randomized SVD

- Preconditioners
- Add preconditioner for symmetric systems
- Preconditioner for least squares

- Performance
- Avoid redoing computations in adaptive sketch
- General performance

- Documentation
- More complete general docs
- Least squares example (sketch & solve, iterative sketching, sketch & precondition)

[1] Zachary Frangella, Joel A Tropp, and Madeleine Udell. “Randomized Nyström Preconditioning.” In:arXiv preprint arXiv:2110.02820(2021). https://arxiv.org/abs/2110.02820

[2] PG Martinsson and JA Tropp. “Randomized numerical linear algebra: foundations & algorithms (2020).” In: arXiv preprint arXiv:2002.01387. https://arxiv.org/abs/2002.01387