# Standard Eigen Decomposition

eigs calculates the eigenvalues and, optionally, eigenvectors of a matrix using implicitly restarted Lanczos or Arnoldi iterations for real symmetric or general nonsymmetric matrices respectively. The input matrix A can be any structured AbstractMatrix that implements the in-place product method LinearAlgebra.mul!(y, A, x).

For the single matrix version,

eigs(A; nev=6, ncv=max(20,2*nev+1), which=:LM, tol=0.0, maxiter=300, sigma=nothing, ritzvec=true, v0=zeros((0,))) -> (d,[v,],nconv,niter,nmult,resid)

the following keyword arguments are supported:

• nev: Number of eigenvalues
• ncv: Number of Krylov vectors used in the computation; should satisfy nev+1 <= ncv <= n for real symmetric problems and nev+2 <= ncv <= n for other problems, where n is the size of the input matrix A. The default is ncv = max(20,2*nev+1). Note that these restrictions limit the input matrix A to be of dimension at least 2.
• which: type of eigenvalues to compute. See the note below.
whichtype of eigenvalues
:LMeigenvalues of largest magnitude (default)
:SMeigenvalues of smallest magnitude
:LReigenvalues of largest real part
:SReigenvalues of smallest real part
:LIeigenvalues of largest imaginary part (nonsymmetric or complex A only)
:SIeigenvalues of smallest imaginary part (nonsymmetric or complex A only)
:BEcompute half of the eigenvalues from each end of the spectrum, biased in favor of the high end. (real symmetric A only)
• tol: parameter defining the relative tolerance for convergence of Ritz values (eigenvalue estimates). A Ritz value $θ$ is considered converged when its associated residual is less than or equal to the product of tol and $max(ɛ^{2/3}, |θ|)$, where ɛ = eps(real(eltype(A)))/2 is LAPACK's machine epsilon. The residual associated with $θ$ and its corresponding Ritz vector $v$ is defined as the norm $||Av - vθ||$. The specified value of tol should be positive; otherwise, it is ignored and $ɛ$ is used instead. Default: $ɛ$.

• maxiter: Maximum number of iterations (default = 300)

• sigma: Specifies the level shift used in inverse iteration. If nothing (default), defaults to ordinary (forward) iterations. Otherwise, find eigenvalues close to sigma using shift and invert iterations.

• ritzvec: Returns the Ritz vectors v (eigenvectors) if true

• v0: starting vector from which to start the iterations

We can see the various keywords in action in the following examples:

julia> A = Diagonal(1:4);

julia> λ, ϕ = eigs(A, nev = 2, which=:SM);

julia> λ
2-element Array{Float64,1}:
1.0000000000000002
2.0

julia> B = Diagonal([1., 2., -3im, 4im]);

julia> λ, ϕ = eigs(B, nev=1, which=:LI);

julia> λ
1-element Array{Complex{Float64},1}:
1.3322676295501878e-15 + 4.0im

julia> λ, ϕ = eigs(B, nev=1, which=:SI);

julia> λ
1-element Array{Complex{Float64},1}:
-2.498001805406602e-16 - 3.0000000000000018im

julia> λ, ϕ = eigs(B, nev=1, which=:LR);

julia> λ
1-element Array{Complex{Float64},1}:
2.0000000000000004 + 4.0615212488780827e-17im

julia> λ, ϕ = eigs(B, nev=1, which=:SR);

julia> λ
1-element Array{Complex{Float64},1}:
-8.881784197001252e-16 + 3.999999999999997im

julia> λ, ϕ = eigs(B, nev=1, sigma=1.5);

julia> λ
1-element Array{Complex{Float64},1}:
1.0000000000000004 + 4.0417078924070745e-18im
Note

The sigma and which keywords interact: the description of eigenvalues searched for by which do not necessarily refer to the eigenvalues of A, but rather the linear operator constructed by the specification of the iteration mode implied by sigma.

sigmaiteration modewhich refers to eigenvalues of
nothingordinary (forward)$A$
real or complexinverse with level shift sigma$(A - \sigma I )^{-1}$
Note

Although tol has a default value, the best choice depends strongly on the matrix A. We recommend that users always specify a value for tol which suits their specific needs.

For details of how the errors in the computed eigenvalues are estimated, see:

• B. N. Parlett, "The Symmetric Eigenvalue Problem", SIAM: Philadelphia, 2/e (1998), Ch. 13.2, "Accessing Accuracy in Lanczos Problems", pp. 290-292 ff.
• R. B. Lehoucq and D. C. Sorensen, "Deflation Techniques for an Implicitly Restarted Arnoldi Iteration", SIAM Journal on Matrix Analysis and Applications (1996), 17(4), 789–821. doi:10.1137/S0895479895281484