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### Computing si, , and , To explain si, , , and in Table 4.7 and Table 4.8, we need to introduce a condition number for an individual eigenvalue, block diagonalization of a matrix pair and the separation of two matrix pairs.

Let be a simple eigenvalue of (A, B) with left and right eigenvectors yi and xi, respectively. si is the reciprocal condition number for a simple eigenvalue of (A, B) : (4.11)

Notice that yHiAxi / yHiBxi is equal to . The condition number si in (4.11) is independent of the normalization of the eigenvectors. In the error bound of Table 4.7 for a simple eigenvalue and in (4.10), si is returned as RCONDE(i) by xGGEVX (as S(i) by xTGSNA).

We assume that the matrix pair (A, B) is in the generalized Schur form. Consider a cluster of m eigenvalues, counting multiplicities. Moreover, assume the n-by-n matrix pair (A, B) is (4.12)

where the eigenvalues of the m-by-m matrix pair (A11, B11) are exactly those in which we are interested. In practice, if the eigenvalues on the (block) diagonal of (A, B) are not in the desired order, routine xTGEXC can be used to put the desired ones in the upper left corner as shown .

An equivalence transformation that block-diagonalizes (A, B) can be expressed as (4.13)

Solving for (L,R) in (4.13) is equivalent to solving the system of linear equations (4.14)

Equation (4.14) is called a generalized Sylvester equation [71,75]. Given the generalized Schur form (4.12), we solve equation (4.14) for L and R using the subroutine xTGSYL. and for the eigenvalues of (A11, B11) are defined as (4.15)

In the perturbation theory for the generalized eigenvalue problem, and play the same role as the norm of the spectral projector |P| does for the standard eigenvalue problem in section 4.8.1.3. Indeed, if B = I, then p = q and p equals the norm of the projection onto an invariant subspace of A. For the generalized eigenvalue problem we need both a left and a right projection norm since the left and right deflating subspaces are (usually) different. In Table 4.8, li and denote the left projector norm corresponding to an individual eigenvalue pair and a cluster of eigenvalues defined by the subset , respectively. Similar notation is used for ri and . The values of and are returned as RCONDE(1) and RCONDE(2) from xGGESX (as PL and PR from xTGSEN).

The separation of two matrix pairs (A11, B11) and (A22, B22) is defined as the smallest singular value of the linear map in (4.14) which takes (L, R) to (A11 R - L A22, B11 R - L B22) : (4.16) is a generalization of the separation between two matrices ( in (4.6)) to two matrix pairs, and it measures the separation of their spectra in the following sense. If (A11, B11) and (A22, B22) have a common eigenvalue, then is zero, and it is small if there is a small perturbation of either (A11, B11) or (A22, B22) that makes them have a common eigenvalue.

Notice that does not generally equal (unless Aii and Bii are symmetric for i = 1, 2). Accordingly, the ordering of the arguments plays a role for the separation of two matrix pairs, while it does not for the separation of two matrices ( ). Therefore, we introduce the notation (4.17)

An associated generalized Sylvester operator (A22 R - L A11, B22 R - L B11) in the definition of is obtained from block-diagonalizing a regular matrix pair in lower block triangular form, just as the operator (A11 R - L A22, B11 R - L B22) in the definition of arises from block-diagonalizing a regular matrix pair (4.12) in upper block triangular form.

In the error bounds of Tables 4.7 and 4.8, and denote , where (A11, B11) corresponds to an individual eigenvalue pair and a cluster of eigenvalues defined by the subset , respectively. Similar notation is used for and . xGGESX reports estimates of and in RCONDV(1) and RCONDV(2) (DIF(1) and DIF(2) in xTGSEN), respectively.

From a matrix representation of (4.14) it is possible to formulate an exact expression of as (4.18)

where Zu is the 2m(n - m)-by-2m(n - m) matrix and is the Kronecker product. A method based directly on forming Zu is generally impractical, since Zu can be as large as n2/2 x n2/2. Thus we would require as much as O(n4) extra workspace and O(n6) operations, much more than the estimation methods that we now describe.

We instead compute an estimate of as the reciprocal value of an estimate of , where Zu is the matrix representation of the generalized Sylvester operator. It is possible to estimate by solving generalized Sylvester equations in triangular form. We provide both Frobenius norm and one norm estimates . The one norm estimate makes the condition estimation uniform with the nonsymmetric eigenvalue problem. The Frobenius norm estimate offers a low cost and equally reliable estimator. The one norm estimate is a factor 3 to 10 times more expensive . From the definition of (4.17) we see that estimates can be computed by using the algorithms for estimating .

Frobenius norm estimate: From the Zux = b representation of the generalized Sylvester equation (4.14) we get a lower bound on : (4.19)

To get an improved estimate we try to choose right hand sides (C, F) such that the associated solution (L, R) has as large norm as possible, giving the estimator (4.20)

Methods for computing such (C, F) are described in [75,74]. The work to compute DIF(1) is comparable to solve a generalized Sylvester equation, which costs only 2m2(n-m) + 2m(n-m)2 operations if the matrix pairs are in generalized Schur form. DIF(2) is the Frobenius norm estimate.

One norm norm estimate: From the relationship (4.21)

we know that can never differ more than a factor from . So it makes sense to compute an one norm estimate of . xLACON implements a method for estimating the one norm of a square matrix, using reverse communication for evaluating matrix and vector products [59,64]. We apply this method to by providing the solution vectors x and y of Zux = z and a transposed system ZuTy = z, where z is determined by xLACON. In each step only one of these generalized Sylvester equations is solved using blocked algorithms . xLACON returns v and such that Zu-1w = v and , resulting in the one-norm-based estimate (4.22)

The cost for computing this bound is roughly equal to the number of steps in the reverse communication times the cost for one generalized Sylvester solve. DIF(2) is the one norm estimate.

The expert driver routines xGGEVX and xGGESX compute the Frobenius norm estimate (4.20). The routine xTGSNA also computes the Frobenius norm estimate (4.20) of and . The routine xTGSEN optionally computes the Frobenius norm estimate (4.20) or the one norm estimate (4.22). The choice of estimate is controlled by the input parameter IJOB.     Next: Singular Eigenproblems Up: Further Details: Error Bounds Previous: Balancing and Conditioning   Contents   Index
Susan Blackford
1999-10-01