## MB02CD

### Cholesky factorization of a positive definite block Toeplitz matrix

[Specification] [Arguments] [Method] [References] [Comments] [Example]

Purpose

      To compute the Cholesky factor and the generator and/or the
      Cholesky factor of the inverse of a symmetric positive definite
      (s.p.d.) block Toeplitz matrix T, defined by either its first
      block row, or its first block column, depending on the routine
      parameter TYPET. Transformation information is stored.

Specification

          SUBROUTINE MB02CD( JOB, TYPET, K, N, T, LDT, G, LDG, R, LDR, L,
         $                   LDL, CS, LCS, DWORK, LDWORK, INFO )
    C     .. Scalar Arguments ..
          CHARACTER         JOB, TYPET
          INTEGER           INFO, K, LCS, LDG, LDL, LDR, LDT, LDWORK, N
    C     .. Array Arguments ..
          DOUBLE PRECISION  CS(*), DWORK(*), G(LDG, *), L(LDL,*), R(LDR,*),
         $                  T(LDT,*)

Arguments

Mode Parameters

      JOB     CHARACTER*1
              Specifies the output of the routine, as follows:
              = 'G':  only computes the generator G of the inverse;
              = 'R':  computes the generator G of the inverse and the
                      Cholesky factor R of T, i.e., if TYPET = 'R',
                      then R'*R = T, and if TYPET = 'C', then R*R' = T;
              = 'L':  computes the generator G and the Cholesky factor L
                      of the inverse, i.e., if TYPET = 'R', then
                      L'*L = inv(T), and if TYPET = 'C', then
                      L*L' = inv(T);
              = 'A':  computes the generator G, the Cholesky factor L
                      of the inverse and the Cholesky factor R of T;
              = 'O':  only computes the Cholesky factor R of T.

      TYPET   CHARACTER*1
              Specifies the type of T, as follows:
              = 'R':  T contains the first block row of an s.p.d. block
                      Toeplitz matrix; if demanded, the Cholesky factors
                      R and L are upper and lower triangular,
                      respectively, and G contains the transposed
                      generator of the inverse;
              = 'C':  T contains the first block column of an s.p.d.
                      block Toeplitz matrix; if demanded, the Cholesky
                      factors R and L are lower and upper triangular,
                      respectively, and G contains the generator of the
                      inverse. This choice results in a column oriented
                      algorithm which is usually faster.
              Note:   in the sequel, the notation x / y means that
                      x corresponds to TYPET = 'R' and y corresponds to
                      TYPET = 'C'.

Input/Output Parameters

      K       (input)  INTEGER
              The number of rows / columns in T, which should be equal
              to the blocksize.  K >= 0.

      N       (input)  INTEGER
              The number of blocks in T.  N >= 0.

      T       (input/output)  DOUBLE PRECISION array, dimension
              (LDT,N*K) / (LDT,K)
              On entry, the leading K-by-N*K / N*K-by-K part of this
              array must contain the first block row / column of an
              s.p.d. block Toeplitz matrix.
              On exit, if INFO = 0, then the leading K-by-N*K / N*K-by-K
              part of this array contains, in the first K-by-K block,
              the upper / lower Cholesky factor of T(1:K,1:K), and in
              the remaining part, the Householder transformations
              applied during the process.

      LDT     INTEGER
              The leading dimension of the array T.
              LDT >= MAX(1,K),    if TYPET = 'R';
              LDT >= MAX(1,N*K),  if TYPET = 'C'.

      G       (output)  DOUBLE PRECISION array, dimension
              (LDG,N*K) / (LDG,2*K)
              If INFO = 0 and JOB = 'G', 'R', 'L', or 'A', the leading
              2*K-by-N*K / N*K-by-2*K part of this array contains, in
              the first K-by-K block of the second block row / column,
              the lower right block of L (necessary for updating
              factorizations in SLICOT Library routine MB02DD), and
              in the remaining part, the generator of the inverse of T.
              Actually, to obtain a generator one has to set
                  G(K+1:2*K, 1:K) = 0,    if TYPET = 'R';
                  G(1:K, K+1:2*K) = 0,    if TYPET = 'C'.

      LDG     INTEGER
              The leading dimension of the array G.
              LDG >= MAX(1,2*K),  if TYPET = 'R' and
                                     JOB = 'G', 'R', 'L', or 'A';
              LDG >= MAX(1,N*K),  if TYPET = 'C' and
                                     JOB = 'G', 'R', 'L', or 'A';
              LDG >= 1,           if JOB = 'O'.

      R       (output)  DOUBLE PRECISION array, dimension (LDR,N*K)
              If INFO = 0 and JOB = 'R', 'A', or 'O', then the leading
              N*K-by-N*K part of this array contains the upper / lower
              Cholesky factor of T.
              The elements in the strictly lower / upper triangular part
              are not referenced.

      LDR     INTEGER
              The leading dimension of the array R.
              LDR >= MAX(1,N*K),  if JOB = 'R', 'A', or 'O';
              LDR >= 1,           if JOB = 'G', or 'L'.

      L       (output)  DOUBLE PRECISION array, dimension (LDL,N*K)
              If INFO = 0 and JOB = 'L', or 'A', then the leading
              N*K-by-N*K part of this array contains the lower / upper
              Cholesky factor of the inverse of T.
              The elements in the strictly upper / lower triangular part
              are not referenced.

      LDL     INTEGER
              The leading dimension of the array L.
              LDL >= MAX(1,N*K),  if JOB = 'L', or 'A';
              LDL >= 1,           if JOB = 'G', 'R', or 'O'.

      CS      (output)  DOUBLE PRECISION array, dimension (LCS)
              If INFO = 0, then the leading 3*(N-1)*K part of this
              array contains information about the hyperbolic rotations
              and Householder transformations applied during the
              process. This information is needed for updating the
              factorizations in SLICOT Library routine MB02DD.

      LCS     INTEGER
              The length of the array CS.  LCS >= 3*(N-1)*K.

Workspace

      DWORK   DOUBLE PRECISION array, dimension (LDWORK)
              On exit, if INFO = 0,  DWORK(1)  returns the optimal
              value of LDWORK.
              On exit, if  INFO = -16,  DWORK(1)  returns the minimum
              value of LDWORK.

      LDWORK  INTEGER
              The length of the array DWORK.
              LDWORK >= MAX(1,(N-1)*K).
              For optimum performance LDWORK should be larger.

Error Indicator

      INFO    INTEGER
              = 0:  successful exit;
              < 0:  if INFO = -i, the i-th argument had an illegal
                    value;
              = 1:  the reduction algorithm failed. The Toeplitz matrix
                    associated with T is not (numerically) positive
                    definite.

Method

      Householder transformations and modified hyperbolic rotations
      are used in the Schur algorithm [1], [2].

References

      [1] Kailath, T. and Sayed, A.
          Fast Reliable Algorithms for Matrices with Structure.
          SIAM Publications, Philadelphia, 1999.

      [2] Kressner, D. and Van Dooren, P.
          Factorizations and linear system solvers for matrices with
          Toeplitz structure.
          SLICOT Working Note 2000-2, 2000.

Numerical Aspects

      The implemented method is numerically stable.
                                3 2
      The algorithm requires 0(K N ) floating point operations.

Further Comments

      None

Example

Program Text

    *     MB02CD EXAMPLE PROGRAM TEXT
    *
    *     .. Parameters ..
          DOUBLE PRECISION ZERO
          PARAMETER        ( ZERO = 0.0D0 )
          INTEGER          NIN, NOUT
          PARAMETER        ( NIN = 5, NOUT = 6 )
          INTEGER          KMAX, NMAX
          PARAMETER        ( KMAX = 20, NMAX = 20 )
          INTEGER          LCS, LDG, LDL, LDR, LDT, LDWORK
          PARAMETER        ( LDG = 2*KMAX, LDL = NMAX*KMAX, LDR = NMAX*KMAX,
         $                   LDT = KMAX, LDWORK = ( NMAX - 1 )*KMAX )
          PARAMETER        ( LCS = 3*LDWORK )
    *     .. Local Scalars ..
          INTEGER          I, INFO, J, K, M, N
          CHARACTER        JOB, TYPET
    *     .. Local Arrays .. (Dimensioned for TYPET = 'R'.)
          DOUBLE PRECISION CS(LCS), DWORK(LDWORK), G(LDG, NMAX*KMAX),
         $                 L(LDL, NMAX*KMAX), R(LDR, NMAX*KMAX),
         $                 T(LDT, NMAX*KMAX)
    *     .. External Functions ..
          LOGICAL          LSAME
          EXTERNAL         LSAME
    *     .. External Subroutines ..
          EXTERNAL         DLASET, MB02CD
    *
    *     .. Executable Statements ..
          WRITE ( NOUT, FMT = 99999 )
    *     Skip the heading in the data file and read the data.
          READ ( NIN, FMT = '()' )
          READ ( NIN, FMT = * ) N, K, JOB
          TYPET = 'R'
          M = N*K
          IF( N.LE.0 .OR. N.GT.NMAX ) THEN
             WRITE ( NOUT, FMT = 99993 ) N
          ELSE
             IF( K.LE.0 .OR. K.GT.KMAX ) THEN
                WRITE ( NOUT, FMT = 99992 ) K
             ELSE
                READ ( NIN, FMT = * ) ( ( T(I,J), J = 1,M ), I = 1,K )
    *           Compute the Cholesky factor(s) and/or the generator.
                CALL MB02CD( JOB, TYPET, K, N, T, LDT, G, LDG, R, LDR, L,
         $                   LDL, CS, LCS, DWORK, LDWORK, INFO )
                IF ( INFO.NE.0 ) THEN
                   WRITE ( NOUT, FMT = 99998 ) INFO
                ELSE
                   IF ( LSAME( JOB, 'G' ) .OR. LSAME( JOB, 'A' ) .OR.
         $              LSAME( JOB, 'L' ) .OR. LSAME( JOB, 'R' ) ) THEN
                      WRITE ( NOUT, FMT = 99997 )
                      CALL DLASET( 'Full', K, K, ZERO, ZERO, G(K+1,1), LDG )
                      DO 10  I = 1, 2*K
                         WRITE ( NOUT, FMT = 99994 ) ( G(I,J), J = 1, M )
       10             CONTINUE
                   END IF
                   IF ( LSAME( JOB, 'L' ) .OR. LSAME( JOB, 'A' ) ) THEN
                      WRITE ( NOUT, FMT = 99996 )
                      DO 20  I = 1, M
                         WRITE ( NOUT, FMT = 99994 ) ( L(I,J), J = 1, M )
       20             CONTINUE
                   END IF
                   IF ( LSAME( JOB, 'R' ) .OR. LSAME( JOB, 'A' )
         $                                .OR. LSAME( JOB, 'O' ) ) THEN
                      WRITE ( NOUT, FMT = 99995 )
                      DO 30  I = 1, M
                         WRITE ( NOUT, FMT = 99994 ) ( R(I,J), J = 1, M )
       30             CONTINUE
                   END IF
                END IF
             END IF
          END IF
          STOP
    *
    99999 FORMAT (' MB02CD EXAMPLE PROGRAM RESULTS',/1X)
    99998 FORMAT (' INFO on exit from MB02CD = ',I2)
    99997 FORMAT (' The generator of the inverse of block Toeplitz matrix',
         $        ' is ')
    99996 FORMAT (/' The lower Cholesky factor of the inverse is ')
    99995 FORMAT (/' The upper Cholesky factor of block Toeplitz matrix is '
         $       )
    99994 FORMAT (20(1X,F8.4))
    99993 FORMAT (/' N is out of range.',/' N = ',I5)
    99992 FORMAT (/' K is out of range.',/' K = ',I5)
          END

Program Data

    MB02CD EXAMPLE PROGRAM DATA
      3    2     A
      3.0000    1.0000    0.1000    0.1000    0.2000    0.0500
      1.0000    4.0000    0.4000    0.1000    0.0400    0.2000

Program Results

     MB02CD EXAMPLE PROGRAM RESULTS

     The generator of the inverse of block Toeplitz matrix is
      -0.2355   0.5231  -0.0642   0.0077   0.0187  -0.0265
      -0.5568  -0.0568   0.0229   0.0060   0.0363   0.0000
       0.0000   0.0000  -0.0387   0.0052   0.0003  -0.0575
       0.0000   0.0000   0.0119  -0.0265  -0.0110   0.0076

     The lower Cholesky factor of the inverse is
       0.5774   0.0000   0.0000   0.0000   0.0000   0.0000
      -0.1741   0.5222   0.0000   0.0000   0.0000   0.0000
       0.0000  -0.0581   0.5812   0.0000   0.0000   0.0000
      -0.0142   0.0080  -0.1747   0.5224   0.0000   0.0000
      -0.0387   0.0052   0.0003  -0.0575   0.5825   0.0000
       0.0119  -0.0265  -0.0110   0.0076  -0.1754   0.5231

     The upper Cholesky factor of block Toeplitz matrix is
       1.7321   0.5774   0.0577   0.0577   0.1155   0.0289
       0.0000   1.9149   0.1915   0.0348  -0.0139   0.0957
       0.0000   0.0000   1.7205   0.5754   0.0558   0.0465
       0.0000   0.0000   0.0000   1.9142   0.1890   0.0357
       0.0000   0.0000   0.0000   0.0000   1.7169   0.5759
       0.0000   0.0000   0.0000   0.0000   0.0000   1.9118

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