In mathematics, the matrix sign function is a matrix function on square matrices analogous to the complex sign function. [1]
It was introduced by J.D. Roberts in 1971 as a tool for model reduction and for solving Lyapunov and Algebraic Riccati equation in a technical report of Cambridge University, which was later published in a journal in 1980. [2] [3]
The matrix sign function is a generalization of the complex signum function
to the matrix valued analogue . Although the sign function is not analytic, the matrix function is well defined for all matrices that have no eigenvalue on the imaginary axis, see for example the Jordan-form-based definition (where the derivatives are all zero).
Theorem: Let , then . [1]
Theorem: Let , then is diagonalizable and has eigenvalues that are . [1]
Theorem: Let , then is a projector onto the invariant subspace associated with the eigenvalues in the right-half plane, and analogously for and the left-half plane. [1]
Theorem: Let , and be a Jordan decomposition such that corresponds to eigenvalues with positive real part and to eigenvalue with negative real part. Then , where and are identity matrices of sizes corresponding to and , respectively. [1]
The function can be computed with generic methods for matrix functions, but there are also specialized methods.
The Newton iteration can be derived by observing that , which in terms of matrices can be written as , where we use the matrix square root. If we apply the Babylonian method to compute the square root of the matrix , that is, the iteration , and define the new iterate , we arrive at the iteration
,
where typically . Convergence is global, and locally it is quadratic. [1] [2]
The Newton iteration uses the explicit inverse of the iterates .
To avoid the need of an explicit inverse used in the Newton iteration, the inverse can be approximated with one step of the Newton iteration for the inverse, , derived by Schulz(de) in 1933. [4] Substituting this approximation into the previous method, the new method becomes
.
Convergence is (still) quadratic, but only local (guaranteed for ). [1]
Theorem: [2] [3] Let and assume that and are stable, then the unique solution to the Sylvester equation, , is given by such that
Proof sketch: The result follows from the similarity transform
since
due to the stability of and .
The theorem is, naturally, also applicable to the Lyapunov equation. However, due to the structure the Newton iteration simplifies to only involving inverses of and .
There is a similar result applicable to the algebraic Riccati equation, . [1] [2] Define as
Under the assumption that are Hermitian and there exists a unique stabilizing solution, in the sense that is stable, that solution is given by the over-determined, but consistent, linear system
Proof sketch: The similarity transform
and the stability of implies that
for some matrix .
The Denman–Beavers iteration for the square root of a matrix can be derived from the Newton iteration for the matrix sign function by noticing that is a degenerate algebraic Riccati equation [3] and by definition a solution is the square root of .
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