**Purpose**

To compute the normal rank of the transfer-function matrix of a state-space model (A,B,C,D).

SUBROUTINE AB08MD( EQUIL, N, M, P, A, LDA, B, LDB, C, LDC, D, LDD, $ RANK, TOL, IWORK, DWORK, LDWORK, INFO ) C .. Scalar Arguments .. CHARACTER EQUIL INTEGER INFO, LDA, LDB, LDC, LDD, LDWORK, M, N, P, RANK DOUBLE PRECISION TOL C .. Array Arguments .. INTEGER IWORK(*) DOUBLE PRECISION A(LDA,*), B(LDB,*), C(LDC,*), D(LDD,*), DWORK(*)

**Mode Parameters**

EQUIL CHARACTER*1 Specifies whether the user wishes to balance the compound matrix (see METHOD) as follows: = 'S': Perform balancing (scaling); = 'N': Do not perform balancing.

N (input) INTEGER The number of state variables, i.e., the order of the matrix A. N >= 0. M (input) INTEGER The number of system inputs. M >= 0. P (input) INTEGER The number of system outputs. P >= 0. A (input) DOUBLE PRECISION array, dimension (LDA,N) The leading N-by-N part of this array must contain the state dynamics matrix A of the system. LDA INTEGER The leading dimension of array A. LDA >= MAX(1,N). B (input) DOUBLE PRECISION array, dimension (LDB,M) The leading N-by-M part of this array must contain the input/state matrix B of the system. LDB INTEGER The leading dimension of array B. LDB >= MAX(1,N). C (input) DOUBLE PRECISION array, dimension (LDC,N) The leading P-by-N part of this array must contain the state/output matrix C of the system. LDC INTEGER The leading dimension of array C. LDC >= MAX(1,P). D (input) DOUBLE PRECISION array, dimension (LDD,M) The leading P-by-M part of this array must contain the direct transmission matrix D of the system. LDD INTEGER The leading dimension of array D. LDD >= MAX(1,P). RANK (output) INTEGER The normal rank of the transfer-function matrix.

TOL DOUBLE PRECISION A tolerance used in rank decisions to determine the effective rank, which is defined as the order of the largest leading (or trailing) triangular submatrix in the QR (or RQ) factorization with column (or row) pivoting whose estimated condition number is less than 1/TOL. If the user sets TOL to be less than SQRT((N+P)*(N+M))*EPS then the tolerance is taken as SQRT((N+P)*(N+M))*EPS, where EPS is the machine precision (see LAPACK Library Routine DLAMCH).

IWORK INTEGER array, dimension (2*N+MAX(M,P)+1) DWORK DOUBLE PRECISION array, dimension (LDWORK) On exit, if INFO = 0, DWORK(1) returns the optimal value of LDWORK. LDWORK INTEGER The length of the array DWORK. LDWORK >= (N+P)*(N+M) + MAX( MIN(P,M) + MAX(3*M-1,N), 1, MIN(P,N) + MAX(3*P-1,N+P,N+M) ) For optimum performance LDWORK should be larger. If LDWORK = -1, then a workspace query is assumed; the routine only calculates the optimal size of the DWORK array, returns this value as the first entry of the DWORK array, and no error message related to LDWORK is issued by XERBLA.

INFO INTEGER = 0: successful exit; < 0: if INFO = -i, the i-th argument had an illegal value.

The routine reduces the (N+P)-by-(M+N) compound matrix (B A) (D C) to one with the same invariant zeros and with D of full row rank. The normal rank of the transfer-function matrix is the rank of D.

[1] Svaricek, F. Computation of the Structural Invariants of Linear Multivariable Systems with an Extended Version of the Program ZEROS. System & Control Letters, 6, pp. 261-266, 1985. [2] Emami-Naeini, A. and Van Dooren, P. Computation of Zeros of Linear Multivariable Systems. Automatica, 18, pp. 415-430, 1982.

The algorithm is backward stable (see [2] and [1]).

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**Program Text**

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