# Eigenvalue problems

## Eigenvalues and eigenvectors

Finding a selection of eigenvalues and corresponding (right) eigenvectors of a linear map can be accomplished with the eigsolve routine:

KrylovKit.eigsolveFunction
eigsolve(A::AbstractMatrix, [x₀, howmany = 1, which = :LM, T = eltype(A)]; kwargs...)
eigsolve(f, n::Int, [howmany = 1, which = :LM, T = Float64]; kwargs...)
eigsolve(f, x₀, [howmany = 1, which = :LM]; kwargs...)
# expert version:
eigsolve(f, x₀, howmany, which, algorithm)

Compute at least howmany eigenvalues from the linear map encoded in the matrix A or by the function f. Return eigenvalues, eigenvectors and a ConvergenceInfo structure.

Arguments:

The linear map can be an AbstractMatrix (dense or sparse) or a general function or callable object. If an AbstractMatrix is used, a starting vector x₀ does not need to be provided, it is then chosen as rand(T, size(A,1)). If the linear map is encoded more generally as a a callable function or method, the best approach is to provide an explicit starting guess x₀. Note that x₀ does not need to be of type AbstractVector; any type that behaves as a vector and supports the required methods (see KrylovKit docs) is accepted. If instead of x₀ an integer n is specified, it is assumed that x₀ is a regular vector and it is initialized to rand(T,n), where the default value of T is Float64, unless specified differently.

The next arguments are optional, but should typically be specified. howmany specifies how many eigenvalues should be computed; which specifies which eigenvalues should be targeted. Valid specifications of which are given by

• :LM: eigenvalues of largest magnitude
• :LR: eigenvalues with largest (most positive) real part
• :SR: eigenvalues with smallest (most negative) real part
• :LI: eigenvalues with largest (most positive) imaginary part, only if T <: Complex
• :SI: eigenvalues with smallest (most negative) imaginary part, only if T <: Complex
• EigSorter(f; rev = false): eigenvalues λ that appear first (or last if rev == true) when sorted by f(λ)
Note about selecting which eigenvalues

Krylov methods work well for extremal eigenvalues, i.e. eigenvalues on the periphery of the spectrum of the linear map. All of they valid Symbols for which have this property, but could also be specified using EigSorter, e.g. :LM is equivalent to Eigsorter(abs; rev = true). Note that smallest magnitude sorting is obtained using e.g. EigSorter(abs; rev = false), but since no (shift-and)-invert is used, this will only be successful if you somehow know that eigenvalues close to zero are also close to the periphery of the spectrum.

Degenerate eigenvalues

From a theoretical point of view, Krylov methods can at most find a single eigenvector associated with a targetted eigenvalue, even if the latter is degenerate. In the case of a degenerate eigenvalue, the specific eigenvector that is returned is determined by the starting vector x₀. For large problems, this turns out to be less of an issue in practice, as often a second linearly independent eigenvector is generated out of the numerical noise resulting from the orthogonalisation steps in the Lanczos or Arnoldi iteration. Nonetheless, it is important to take this into account and to try not to depend on this potentially fragile behaviour, especially for smaller problems.

The argument T acts as a hint in which Number type the computation should be performed, but is not restrictive. If the linear map automatically produces complex values, complex arithmetic will be used even though T<:Real was specified. However, if the linear map and initial guess are real, approximate eigenvalues will be searched for using a partial Schur factorization, which implies that complex conjugate eigenvalues come in pairs and cannot be split. It is then illegal to choose which in a way that would treat λ and conj(λ) differently, i.e. :LI and :SI are invalid, as well as any EigSorter that would lead to by(λ) != by(conj(λ)).

Return values:

The return value is always of the form vals, vecs, info = eigsolve(...) with

• vals: a Vector containing the eigenvalues, of length at least howmany, but could be longer if more eigenvalues were converged at the same cost. Eigenvalues will be real if Lanczos was used and complex if Arnoldi was used (see below).

• vecs: a Vector of corresponding eigenvectors, of the same length as vals. Note that eigenvectors are not returned as a matrix, as the linear map could act on any custom Julia type with vector like behavior, i.e. the elements of the list vecs are objects that are typically similar to the starting guess x₀, up to a possibly different eltype. In particular for a general matrix (i.e. with Arnoldi) the eigenvectors are generally complex and are therefore always returned in a complex number format. When the linear map is a simple AbstractMatrix, vecs will be Vector{Vector{<:Number}}.

• info: an object of type [ConvergenceInfo], which has the following fields

• info.converged::Int: indicates how many eigenvalues and eigenvectors were actually converged to the specified tolerance tol (see below under keyword arguments)
• info.residual::Vector: a list of the same length as vals containing the residuals info.residual[i] = f(vecs[i]) - vals[i] * vecs[i]
• info.normres::Vector{<:Real}: list of the same length as vals containing the norm of the residual info.normres[i] = norm(info.residual[i])
• info.numops::Int: number of times the linear map was applied, i.e. number of times f was called, or a vector was multiplied with A
• info.numiter::Int: number of times the Krylov subspace was restarted (see below)
Check for convergence

No warning is printed if not all requested eigenvalues were converged, so always check if info.converged >= howmany.

Keyword arguments:

Keyword arguments and their default values are given by:

• verbosity::Int = 0: verbosity level, i.e. 0 (no messages), 1 (single message at the end), 2 (information after every iteration), 3 (information per Krylov step)
• tol::Real: the requested accuracy (corresponding to the 2-norm of the residual for Schur vectors, not the eigenvectors). If you work in e.g. single precision (Float32), you should definitely change the default value.
• krylovdim::Integer: the maximum dimension of the Krylov subspace that will be constructed. Note that the dimension of the vector space is not known or checked, e.g. x₀ should not necessarily support the Base.length function. If you know the actual problem dimension is smaller than the default value, it is useful to reduce the value of krylovdim, though in principle this should be detected.
• maxiter::Integer: the number of times the Krylov subspace can be rebuilt; see below for further details on the algorithms.
• orth::Orthogonalizer: the orthogonalization method to be used, see Orthogonalizer
• issymmetric::Bool: if the linear map is symmetric, only meaningful if T<:Real
• ishermitian::Bool: if the linear map is hermitian
• eager::Bool = false: if true, eagerly compute the eigenvalue or Schur decomposition after every expansion of the Krylov subspace to test for convergence, otherwise wait until the Krylov subspace has dimension krylovdim. This can result in a faster return, for example if the initial guess is very good, but also has some overhead, as many more dense Schur factorizations need to be computed.

The default values are given by tol = KrylovDefaults.tol, krylovdim = KrylovDefaults.krylovdim, maxiter = KrylovDefaults.maxiter, orth = KrylovDefaults.orth; see KrylovDefaults for details.

The default value for the last two parameters depends on the method. If an AbstractMatrix is used, issymmetric and ishermitian are checked for that matrix, otherwise the default values are issymmetric = false and ishermitian = T <: Real && issymmetric. When values for the keyword arguments are provided, no checks will be performed even in the matrix case.

Algorithm

The final (expert) method, without default values and keyword arguments, is the one that is finally called, and can also be used directly. Here, one specifies the algorithm explicitly as either Lanczos, for real symmetric or complex hermitian problems, or Arnoldi, for general problems. Note that these names refer to the process for building the Krylov subspace, but the actual algorithm is an implementation of the Krylov-Schur algorithm, which can dynamically shrink and grow the Krylov subspace, i.e. the restarts are so-called thick restarts where a part of the current Krylov subspace is kept.

In case of a general problem, where the Arnoldi method is used, convergence of an eigenvalue is not based on the norm of the residual norm(f(vecs[i]) - vals[i]*vecs[i]) for the eigenvector but rather on the norm of the residual for the corresponding Schur vectors.

See also schursolve if you want to use the partial Schur decomposition directly, or if you are not interested in computing the eigenvectors, and want to work in real arithmetic all the way true (if the linear map and starting guess are real).

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Which eigenvalues are targeted can be specified using one of the symbols :LM, :LR, :SR, :LI and :SI for largest magnitude, largest and smallest real part, and largest and smallest imaginary part respectively. Alternatively, one can just specify a general sorting operation using EigSorter

For a general matrix, eigenvalues and eigenvectors will always be returned with complex values for reasons of type stability. However, if the linear map and initial guess are real, most of the computation is actually performed using real arithmetic, as in fact the first step is to compute an approximate partial Schur factorization. If one is not interested in the eigenvectors, one can also just compute this partial Schur factorization using schursolve, for which only an 'expert' method call is available

KrylovKit.schursolveFunction
# expert version:
schursolve(f, x₀, howmany, which, algorithm)

Compute a partial Schur decomposition containing howmany eigenvalues from the linear map encoded in the matrix or function A. Return the reduced Schur matrix, the basis of Schur vectors, the extracted eigenvalues and a ConvergenceInfo structure.

See also eigsolve to obtain the eigenvectors instead. For real symmetric or complex hermitian problems, the (partial) Schur decomposition is identical to the (partial) eigenvalue decomposition, and eigsolve should always be used.

Arguments:

The linear map can be an AbstractMatrix (dense or sparse) or a general function or callable object, that acts on vector like objects similar to x₀, which is the starting guess from which a Krylov subspace will be built. howmany specifies how many Schur vectors should be converged before the algorithm terminates; which specifies which eigenvalues should be targeted. Valid specifications of which are

• LM: eigenvalues of largest magnitude
• LR: eigenvalues with largest (most positive) real part
• SR: eigenvalues with smallest (most negative) real part
• LI: eigenvalues with largest (most positive) imaginary part, only if T <: Complex
• SI: eigenvalues with smallest (most negative) imaginary part, only if T <: Complex
• EigSorter(f; rev = false): eigenvalues λ that appear first (or last if rev == true) when sorted by f(λ)
Note about selecting which eigenvalues

Krylov methods work well for extremal eigenvalues, i.e. eigenvalues on the periphery of the spectrum of the linear map. All of they valid Symbols for which have this property, but could also be specified using EigSorter, e.g. :LM is equivalent to Eigsorter(abs; rev = true). Note that smallest magnitude sorting is obtained using e.g. EigSorter(abs; rev = false), but since no (shift-and)-invert is used, this will only be successful if you somehow know that eigenvalues close to zero are also close to the periphery of the spectrum.

The final argument algorithm can currently only be an instance of Arnoldi, but should nevertheless be specified. Since schursolve is less commonly used as eigsolve, no convenient keyword syntax is currently available.

Return values:

The return value is always of the form T, vecs, vals, info = schursolve(...) with

• T: a Matrix containing the partial Schur decomposition of the linear map, i.e. it's elements are given by T[i,j] = dot(vecs[i], f(vecs[j])). It is of Schur form, i.e. upper triangular in case of complex arithmetic, and block upper triangular (with at most 2x2 blocks) in case of real arithmetic.

• vecs: a Vector of corresponding Schur vectors, of the same length as vals. Note that Schur vectors are not returned as a matrix, as the linear map could act on any custom Julia type with vector like behavior, i.e. the elements of the list vecs are objects that are typically similar to the starting guess x₀, up to a possibly different eltype. When the linear map is a simple AbstractMatrix, vecs will be Vector{Vector{<:Number}}. Schur vectors are by definition orthogonal, i.e. dot(vecs[i],vecs[j]) = I[i,j]. Note that Schur vectors are real if the problem (i.e. the linear map and the initial guess) are real.

• vals: a Vector of eigenvalues, i.e. the diagonal elements of T in case of complex arithmetic, or extracted from the diagonal blocks in case of real arithmetic. Note that vals will always be complex, independent of the underlying arithmetic.

• info: an object of type [ConvergenceInfo], which has the following fields

• info.converged::Int: indicates how many eigenvalues and Schur vectors were actually converged to the specified tolerance (see below under keyword arguments)

• info.residuals::Vector: a list of the same length as vals containing the actual residuals

info.residuals[i] = f(vecs[i]) - sum(vecs[j] * T[j, i] for j in 1:i+1)

where T[i+1,i] is definitely zero in case of complex arithmetic and possibly zero in case of real arithmetic

• info.normres::Vector{<:Real}: list of the same length as vals containing the norm of the residual for every Schur vector, i.e. info.normes[i] = norm(info.residual[i])

• info.numops::Int: number of times the linear map was applied, i.e. number of times f was called, or a vector was multiplied with A

• info.numiter::Int: number of times the Krylov subspace was restarted (see below)

Check for convergence

No warning is printed if not all requested eigenvalues were converged, so always check if info.converged >= howmany.

Algorithm

The actual algorithm is an implementation of the Krylov-Schur algorithm, where the Arnoldi algorithm is used to generate the Krylov subspace. During the algorithm, the Krylov subspace is dynamically grown and shrunk, i.e. the restarts are so-called thick restarts where a part of the current Krylov subspace is kept.

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Note that, for symmetric or hermitian linear maps, the eigenvalue and Schur factorization are equivalent, and one should only use eigsolve. There is no schursolve using the Lanczos algorithm.

Another example of a possible use case of schursolve is if the linear map is known to have a unique eigenvalue of, e.g. largest magnitude. Then, if the linear map is real valued, that largest magnitude eigenvalue and its corresponding eigenvector are also real valued. eigsolve will automatically return complex valued eigenvectors for reasons of type stability. However, as the first Schur vector will coincide with the first eigenvector, one can instead use

T, vecs, vals, info = schursolve(A, x⁠₀, 1, :LM, Arnoldi(...))

and use vecs[1] as the real valued eigenvector (after checking info.converged) corresponding to the largest magnitude eigenvalue of A.

## Generalized eigenvalue problems

Generalized eigenvalues λ and corresponding vectors x of the generalized eigenvalue problem $A x = λ B x$ can be obtained using the method geneigsolve. Currently, there is only one algorithm, which does not require inverses of A or B, but is restricted to symmetric or hermitian generalized eigenvalue problems where the matrix or linear map B is positive definite. Note that this is not reflected in the default values for the keyword arguments issymmetric, ishermitian and isposdef, so that these should be set explicitly in order to comply with this restriction. If A and B are actual instances of AbstractMatrix, the default value for the keyword arguments will try to check these properties explicitly.

KrylovKit.geneigsolveFunction
geneigsolve((A::AbstractMatrix, B::AbstractMatrix), [howmany = 1, which = :LM,
T = promote_type(eltype(A), eltype(B))]; kwargs...)
geneigsolve(f, n::Int, [howmany = 1, which = :LM, T = Float64]; kwargs...)
geneigsolve(f, x₀, [howmany = 1, which = :LM]; kwargs...)
# expert version:
geneigsolve(f, x₀, howmany, which, algorithm)

Compute at least howmany generalized eigenvalues $λ$ and generalized eigenvectors $x$ of the form $(A - λB)x = 0$, where A and B are either instances of AbstractMatrix, or some function that implements the matrix vector product. In case functions are used, one could either specify the action of A and B via a tuple of two functions (or a function and an AbstractMatrix), or one could use a single function that takes a single argument x and returns two results, corresponding to A*x and B*x. Return the computed eigenvalues, eigenvectors and a ConvergenceInfo structure.

Arguments:

The first argument is either a tuple of two linear maps, so a function or an AbstractMatrix for either of them, representing the action of A and B. Alternatively, a single function can be used that takes a single argument x and returns the equivalent of (A*x, B*x) as result. This latter form is compatible with the do block syntax of Julia. If an AbstractMatrix is used for either A or B, a starting vector x₀ does not need to be provided, it is then chosen as rand(T, size(A,1)) if A is an AbstractMatrix (or similarly if only B is an AbstractMatrix). Here T = promote_type(eltype(A), eltype(B)) if both A and B are instances of AbstractMatrix, or just the eltype of whichever is an AbstractMatrix. If both A and B are encoded more generally as a callable function or method, the best approach is to provide an explicit starting guess x₀. Note that x₀ does not need to be of type AbstractVector, any type that behaves as a vector and supports the required methods (see KrylovKit docs) is accepted. If instead of x₀ an integer n is specified, it is assumed that x₀ is a regular vector and it is initialized to rand(T,n), where the default value of T is Float64, unless specified differently.

The next arguments are optional, but should typically be specified. howmany specifies how many eigenvalues should be computed; which specifies which eigenvalues should be targeted. Valid specifications of which are given by

• :LM: eigenvalues of largest magnitude
• :LR: eigenvalues with largest (most positive) real part
• :SR: eigenvalues with smallest (most negative) real part
• :LI: eigenvalues with largest (most positive) imaginary part, only if T <: Complex
• :SI: eigenvalues with smallest (most negative) imaginary part, only if T <: Complex
• EigSorter(f; rev = false): eigenvalues λ that appear first (or last if rev == true) when sorted by f(λ)
Note about selecting which eigenvalues

Krylov methods work well for extremal eigenvalues, i.e. eigenvalues on the periphery of the spectrum of the linear map. Even with ClosestTo, no shift and invert is performed. This is useful if, e.g., you know the spectrum to be within the unit circle in the complex plane, and want to target the eigenvalues closest to the value λ = 1.

The argument T acts as a hint in which Number type the computation should be performed, but is not restrictive. If the linear map automatically produces complex values, complex arithmetic will be used even though T<:Real was specified.

Return values:

The return value is always of the form vals, vecs, info = geneigsolve(...) with

• vals: a Vector containing the eigenvalues, of length at least howmany, but could be longer if more eigenvalues were converged at the same cost.

• vecs: a Vector of corresponding eigenvectors, of the same length as vals. Note that eigenvectors are not returned as a matrix, as the linear map could act on any custom Julia type with vector like behavior, i.e. the elements of the list vecs are objects that are typically similar to the starting guess x₀, up to a possibly different eltype. When the linear map is a simple AbstractMatrix, vecs will be Vector{Vector{<:Number}}.

• info: an object of type [ConvergenceInfo], which has the following fields

• info.converged::Int: indicates how many eigenvalues and eigenvectors were actually converged to the specified tolerance tol (see below under keyword arguments)
• info.residual::Vector: a list of the same length as vals containing the residuals info.residual[i] = f(vecs[i]) - vals[i] * vecs[i]
• info.normres::Vector{<:Real}: list of the same length as vals containing the norm of the residual info.normres[i] = norm(info.residual[i])
• info.numops::Int: number of times the linear map was applied, i.e. number of times f was called, or a vector was multiplied with A
• info.numiter::Int: number of times the Krylov subspace was restarted (see below)
Check for convergence

No warning is printed if not all requested eigenvalues were converged, so always check if info.converged >= howmany.

Keyword arguments:

Keyword arguments and their default values are given by:

• verbosity::Int = 0: verbosity level, i.e. 0 (no messages), 1 (single message at the end), 2 (information after every iteration), 3 (information per Krylov step)
• tol::Real: the requested accuracy (corresponding to the 2-norm of the residual for Schur vectors, not the eigenvectors). If you work in e.g. single precision (Float32), you should definitely change the default value.
• krylovdim::Integer: the maximum dimension of the Krylov subspace that will be constructed. Note that the dimension of the vector space is not known or checked, e.g. x₀ should not necessarily support the Base.length function. If you know the actual problem dimension is smaller than the default value, it is useful to reduce the value of krylovdim, though in principle this should be detected.
• maxiter::Integer: the number of times the Krylov subspace can be rebuilt; see below for further details on the algorithms.
• orth::Orthogonalizer: the orthogonalization method to be used, see Orthogonalizer
• issymmetric::Bool: if both linear maps A and B are symmetric, only meaningful if T<:Real
• ishermitian::Bool: if both linear maps A and B are hermitian
• isposdef::Bool: if the linear map B is positive definite The default values are given by tol = KrylovDefaults.tol, krylovdim = KrylovDefaults.krylovdim, maxiter = KrylovDefaults.maxiter, orth = KrylovDefaults.orth; see KrylovDefaults for details.

The default value for the last three parameters depends on the method. If an AbstractMatrix is used, issymmetric, ishermitian and isposdef are checked for that matrix, otherwise the default values are issymmetric = false and ishermitian = T <: Real && issymmetric. When values are provided, no checks will be performed even in the matrix case.

Algorithm

The last method, without default values and keyword arguments, is the one that is finally called, and can also be used directly. Here the algorithm is specified, though currently only GolubYe is available. The Golub-Ye algorithm is an algorithm for solving hermitian (symmetric) generalized eigenvalue problems A x = λ B x with positive definite B, without the need for inverting B. It builds a Krylov subspace of size krylovdim starting from an estimate x by acting with (A - ρ(x) B), where ρ(x) = dot(x, A*x)/ dot(x, B*x), and employing the Lanczos algorithm. This process is repeated at most maxiter times. In every iteration k>1, the subspace will also be expanded to size krylovdim+1 by adding $x_k - x_{k-1}$, which is known as the LOPCG correction and was suggested by Money and Ye. With krylovdim = 2, this algorithm becomes equivalent to LOPCG.

Restriction to symmetric definite generalized eigenvalue problems

While the only algorithm so far is restricted to symmetric/hermitian generalized eigenvalue problems with positive definite B, this is not reflected in the default values for the keyword arguments issymmetric or ishermitian and isposdef. Make sure to set these to true to understand the implications of using this algorithm.

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