LAPACK: SRC/sgetri.f Source File

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00001       SUBROUTINE SGETRI( N, A, LDA, IPIV, WORK, LWORK, INFO )
00002 *
00003 *  -- LAPACK routine (version 3.2) --
00004 *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
00005 *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
00006 *     November 2006
00007 *
00008 *     .. Scalar Arguments ..
00009       INTEGER            INFO, LDA, LWORK, N
00010 *     ..
00011 *     .. Array Arguments ..
00012       INTEGER            IPIV( * )
00013       REAL               A( LDA, * ), WORK( * )
00014 *     ..
00015 *
00016 *  Purpose
00017 *  =======
00018 *
00019 *  SGETRI computes the inverse of a matrix using the LU factorization
00020 *  computed by SGETRF.
00021 *
00022 *  This method inverts U and then computes inv(A) by solving the system
00023 *  inv(A)*L = inv(U) for inv(A).
00024 *
00025 *  Arguments
00026 *  =========
00027 *
00028 *  N       (input) INTEGER
00029 *          The order of the matrix A.  N >= 0.
00030 *
00031 *  A       (input/output) REAL array, dimension (LDA,N)
00032 *          On entry, the factors L and U from the factorization
00033 *          A = P*L*U as computed by SGETRF.
00034 *          On exit, if INFO = 0, the inverse of the original matrix A.
00035 *
00036 *  LDA     (input) INTEGER
00037 *          The leading dimension of the array A.  LDA >= max(1,N).
00038 *
00039 *  IPIV    (input) INTEGER array, dimension (N)
00040 *          The pivot indices from SGETRF; for 1<=i<=N, row i of the
00041 *          matrix was interchanged with row IPIV(i).
00042 *
00043 *  WORK    (workspace/output) REAL array, dimension (MAX(1,LWORK))
00044 *          On exit, if INFO=0, then WORK(1) returns the optimal LWORK.
00045 *
00046 *  LWORK   (input) INTEGER
00047 *          The dimension of the array WORK.  LWORK >= max(1,N).
00048 *          For optimal performance LWORK >= N*NB, where NB is
00049 *          the optimal blocksize returned by ILAENV.
00050 *
00051 *          If LWORK = -1, then a workspace query is assumed; the routine
00052 *          only calculates the optimal size of the WORK array, returns
00053 *          this value as the first entry of the WORK array, and no error
00054 *          message related to LWORK is issued by XERBLA.
00055 *
00056 *  INFO    (output) INTEGER
00057 *          = 0:  successful exit
00058 *          < 0:  if INFO = -i, the i-th argument had an illegal value
00059 *          > 0:  if INFO = i, U(i,i) is exactly zero; the matrix is
00060 *                singular and its inverse could not be computed.
00061 *
00062 *  =====================================================================
00063 *
00064 *     .. Parameters ..
00065       REAL               ZERO, ONE
00066       PARAMETER          ( ZERO = 0.0E+0, ONE = 1.0E+0 )
00067 *     ..
00068 *     .. Local Scalars ..
00069       LOGICAL            LQUERY
00070       INTEGER            I, IWS, J, JB, JJ, JP, LDWORK, LWKOPT, NB,
00071      $                   NBMIN, NN
00072 *     ..
00073 *     .. External Functions ..
00074       INTEGER            ILAENV
00075       EXTERNAL           ILAENV
00076 *     ..
00077 *     .. External Subroutines ..
00078       EXTERNAL           SGEMM, SGEMV, SSWAP, STRSM, STRTRI, XERBLA
00079 *     ..
00080 *     .. Intrinsic Functions ..
00081       INTRINSIC          MAX, MIN
00082 *     ..
00083 *     .. Executable Statements ..
00084 *
00085 *     Test the input parameters.
00086 *
00087       INFO = 0
00088       NB = ILAENV( 1, 'SGETRI', ' ', N, -1, -1, -1 )
00089       LWKOPT = N*NB
00090       WORK( 1 ) = LWKOPT
00091       LQUERY = ( LWORK.EQ.-1 )
00092       IF( N.LT.0 ) THEN
00093          INFO = -1
00094       ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
00095          INFO = -3
00096       ELSE IF( LWORK.LT.MAX( 1, N ) .AND. .NOT.LQUERY ) THEN
00097          INFO = -6
00098       END IF
00099       IF( INFO.NE.0 ) THEN
00100          CALL XERBLA( 'SGETRI', -INFO )
00101          RETURN
00102       ELSE IF( LQUERY ) THEN
00103          RETURN
00104       END IF
00105 *
00106 *     Quick return if possible
00107 *
00108       IF( N.EQ.0 )
00109      $   RETURN
00110 *
00111 *     Form inv(U).  If INFO > 0 from STRTRI, then U is singular,
00112 *     and the inverse is not computed.
00113 *
00114       CALL STRTRI( 'Upper', 'Non-unit', N, A, LDA, INFO )
00115       IF( INFO.GT.0 )
00116      $   RETURN
00117 *
00118       NBMIN = 2
00119       LDWORK = N
00120       IF( NB.GT.1 .AND. NB.LT.N ) THEN
00121          IWS = MAX( LDWORK*NB, 1 )
00122          IF( LWORK.LT.IWS ) THEN
00123             NB = LWORK / LDWORK
00124             NBMIN = MAX( 2, ILAENV( 2, 'SGETRI', ' ', N, -1, -1, -1 ) )
00125          END IF
00126       ELSE
00127          IWS = N
00128       END IF
00129 *
00130 *     Solve the equation inv(A)*L = inv(U) for inv(A).
00131 *
00132       IF( NB.LT.NBMIN .OR. NB.GE.N ) THEN
00133 *
00134 *        Use unblocked code.
00135 *
00136          DO 20 J = N, 1, -1
00137 *
00138 *           Copy current column of L to WORK and replace with zeros.
00139 *
00140             DO 10 I = J + 1, N
00141                WORK( I ) = A( I, J )
00142                A( I, J ) = ZERO
00143    10       CONTINUE
00144 *
00145 *           Compute current column of inv(A).
00146 *
00147             IF( J.LT.N )
00148      $         CALL SGEMV( 'No transpose', N, N-J, -ONE, A( 1, J+1 ),
00149      $                     LDA, WORK( J+1 ), 1, ONE, A( 1, J ), 1 )
00150    20    CONTINUE
00151       ELSE
00152 *
00153 *        Use blocked code.
00154 *
00155          NN = ( ( N-1 ) / NB )*NB + 1
00156          DO 50 J = NN, 1, -NB
00157             JB = MIN( NB, N-J+1 )
00158 *
00159 *           Copy current block column of L to WORK and replace with
00160 *           zeros.
00161 *
00162             DO 40 JJ = J, J + JB - 1
00163                DO 30 I = JJ + 1, N
00164                   WORK( I+( JJ-J )*LDWORK ) = A( I, JJ )
00165                   A( I, JJ ) = ZERO
00166    30          CONTINUE
00167    40       CONTINUE
00168 *
00169 *           Compute current block column of inv(A).
00170 *
00171             IF( J+JB.LE.N )
00172      $         CALL SGEMM( 'No transpose', 'No transpose', N, JB,
00173      $                     N-J-JB+1, -ONE, A( 1, J+JB ), LDA,
00174      $                     WORK( J+JB ), LDWORK, ONE, A( 1, J ), LDA )
00175             CALL STRSM( 'Right', 'Lower', 'No transpose', 'Unit', N, JB,
00176      $                  ONE, WORK( J ), LDWORK, A( 1, J ), LDA )
00177    50    CONTINUE
00178       END IF
00179 *
00180 *     Apply column interchanges.
00181 *
00182       DO 60 J = N - 1, 1, -1
00183          JP = IPIV( J )
00184          IF( JP.NE.J )
00185      $      CALL SSWAP( N, A( 1, J ), 1, A( 1, JP ), 1 )
00186    60 CONTINUE
00187 *
00188       WORK( 1 ) = IWS
00189       RETURN
00190 *
00191 *     End of SGETRI
00192 *
00193       END