zgetf2.f.html

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      SUBROUTINE <a name="ZGETF2.1"></a><a href="zgetf2.f.html#ZGETF2.1">ZGETF2</a>( M, N, A, LDA, IPIV, INFO )
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  -- LAPACK routine (version 3.1) --
</span><span class="comment">*</span><span class="comment">     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
</span><span class="comment">*</span><span class="comment">     November 2006
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     .. Scalar Arguments ..
</span>      INTEGER            INFO, LDA, M, N
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">     .. Array Arguments ..
</span>      INTEGER            IPIV( * )
      COMPLEX*16         A( LDA, * )
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  Purpose
</span><span class="comment">*</span><span class="comment">  =======
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  <a name="ZGETF2.18"></a><a href="zgetf2.f.html#ZGETF2.1">ZGETF2</a> computes an LU factorization of a general m-by-n matrix A
</span><span class="comment">*</span><span class="comment">  using partial pivoting with row interchanges.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  The factorization has the form
</span><span class="comment">*</span><span class="comment">     A = P * L * U
</span><span class="comment">*</span><span class="comment">  where P is a permutation matrix, L is lower triangular with unit
</span><span class="comment">*</span><span class="comment">  diagonal elements (lower trapezoidal if m &gt; n), and U is upper
</span><span class="comment">*</span><span class="comment">  triangular (upper trapezoidal if m &lt; n).
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  This is the right-looking Level 2 BLAS version of the algorithm.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  Arguments
</span><span class="comment">*</span><span class="comment">  =========
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  M       (input) INTEGER
</span><span class="comment">*</span><span class="comment">          The number of rows of the matrix A.  M &gt;= 0.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  N       (input) INTEGER
</span><span class="comment">*</span><span class="comment">          The number of columns of the matrix A.  N &gt;= 0.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  A       (input/output) COMPLEX*16 array, dimension (LDA,N)
</span><span class="comment">*</span><span class="comment">          On entry, the m by n matrix to be factored.
</span><span class="comment">*</span><span class="comment">          On exit, the factors L and U from the factorization
</span><span class="comment">*</span><span class="comment">          A = P*L*U; the unit diagonal elements of L are not stored.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  LDA     (input) INTEGER
</span><span class="comment">*</span><span class="comment">          The leading dimension of the array A.  LDA &gt;= max(1,M).
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  IPIV    (output) INTEGER array, dimension (min(M,N))
</span><span class="comment">*</span><span class="comment">          The pivot indices; for 1 &lt;= i &lt;= min(M,N), row i of the
</span><span class="comment">*</span><span class="comment">          matrix was interchanged with row IPIV(i).
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  INFO    (output) INTEGER
</span><span class="comment">*</span><span class="comment">          = 0: successful exit
</span><span class="comment">*</span><span class="comment">          &lt; 0: if INFO = -k, the k-th argument had an illegal value
</span><span class="comment">*</span><span class="comment">          &gt; 0: if INFO = k, U(k,k) is exactly zero. The factorization
</span><span class="comment">*</span><span class="comment">               has been completed, but the factor U is exactly
</span><span class="comment">*</span><span class="comment">               singular, and division by zero will occur if it is used
</span><span class="comment">*</span><span class="comment">               to solve a system of equations.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  =====================================================================
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     .. Parameters ..
</span>      COMPLEX*16         ONE, ZERO
      PARAMETER          ( ONE = ( 1.0D+0, 0.0D+0 ),
     $                   ZERO = ( 0.0D+0, 0.0D+0 ) )
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">     .. Local Scalars ..
</span>      DOUBLE PRECISION   SFMIN
      INTEGER            I, J, JP
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">     .. External Functions ..
</span>      DOUBLE PRECISION   <a name="DLAMCH.70"></a><a href="dlamch.f.html#DLAMCH.1">DLAMCH</a>
      INTEGER            IZAMAX
      EXTERNAL           <a name="DLAMCH.72"></a><a href="dlamch.f.html#DLAMCH.1">DLAMCH</a>, IZAMAX
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">     .. External Subroutines ..
</span>      EXTERNAL           <a name="XERBLA.75"></a><a href="xerbla.f.html#XERBLA.1">XERBLA</a>, ZGERU, ZSCAL, ZSWAP
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">     .. Intrinsic Functions ..
</span>      INTRINSIC          MAX, MIN
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">     .. Executable Statements ..
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     Test the input parameters.
</span><span class="comment">*</span><span class="comment">
</span>      INFO = 0
      IF( M.LT.0 ) THEN
         INFO = -1
      ELSE IF( N.LT.0 ) THEN
         INFO = -2
      ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
         INFO = -4
      END IF
      IF( INFO.NE.0 ) THEN
         CALL <a name="XERBLA.93"></a><a href="xerbla.f.html#XERBLA.1">XERBLA</a>( <span class="string">'<a name="ZGETF2.93"></a><a href="zgetf2.f.html#ZGETF2.1">ZGETF2</a>'</span>, -INFO )
         RETURN
      END IF
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     Quick return if possible
</span><span class="comment">*</span><span class="comment">
</span>      IF( M.EQ.0 .OR. N.EQ.0 )
     $   RETURN
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     Compute machine safe minimum
</span><span class="comment">*</span><span class="comment">
</span>      SFMIN = <a name="DLAMCH.104"></a><a href="dlamch.f.html#DLAMCH.1">DLAMCH</a>(<span class="string">'S'</span>) 
<span class="comment">*</span><span class="comment">
</span>      DO 10 J = 1, MIN( M, N )
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">        Find pivot and test for singularity.
</span><span class="comment">*</span><span class="comment">
</span>         JP = J - 1 + IZAMAX( M-J+1, A( J, J ), 1 )
         IPIV( J ) = JP
         IF( A( JP, J ).NE.ZERO ) THEN
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">           Apply the interchange to columns 1:N.
</span><span class="comment">*</span><span class="comment">
</span>            IF( JP.NE.J )
     $         CALL ZSWAP( N, A( J, 1 ), LDA, A( JP, 1 ), LDA )
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">           Compute elements J+1:M of J-th column.
</span><span class="comment">*</span><span class="comment">
</span>            IF( J.LT.M ) THEN
               IF( ABS(A( J, J )) .GE. SFMIN ) THEN
                  CALL ZSCAL( M-J, ONE / A( J, J ), A( J+1, J ), 1 )
               ELSE
                  DO 20 I = 1, M-J
                     A( J+I, J ) = A( J+I, J ) / A( J, J )
   20             CONTINUE
               END IF
            END IF
<span class="comment">*</span><span class="comment">
</span>         ELSE IF( INFO.EQ.0 ) THEN
<span class="comment">*</span><span class="comment">
</span>            INFO = J
         END IF
<span class="comment">*</span><span class="comment">
</span>         IF( J.LT.MIN( M, N ) ) THEN
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">           Update trailing submatrix.
</span><span class="comment">*</span><span class="comment">
</span>            CALL ZGERU( M-J, N-J, -ONE, A( J+1, J ), 1, A( J, J+1 ),
     $                  LDA, A( J+1, J+1 ), LDA )
         END IF
   10 CONTINUE
      RETURN
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     End of <a name="ZGETF2.146"></a><a href="zgetf2.f.html#ZGETF2.1">ZGETF2</a>
</span><span class="comment">*</span><span class="comment">
</span>      END

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