File:  [local] / rpl / lapack / lapack / zsptrf.f
Revision 1.18: download - view: text, annotated - select for diffs - revision graph
Mon Aug 7 08:39:37 2023 UTC (8 months, 3 weeks ago) by bertrand
Branches: MAIN
CVS tags: rpl-4_1_35, rpl-4_1_34, HEAD
Première mise à jour de lapack et blas.

    1: *> \brief \b ZSPTRF
    2: *
    3: *  =========== DOCUMENTATION ===========
    4: *
    5: * Online html documentation available at
    6: *            http://www.netlib.org/lapack/explore-html/
    7: *
    8: *> \htmlonly
    9: *> Download ZSPTRF + dependencies
   10: *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/zsptrf.f">
   11: *> [TGZ]</a>
   12: *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/zsptrf.f">
   13: *> [ZIP]</a>
   14: *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/zsptrf.f">
   15: *> [TXT]</a>
   16: *> \endhtmlonly
   17: *
   18: *  Definition:
   19: *  ===========
   20: *
   21: *       SUBROUTINE ZSPTRF( UPLO, N, AP, IPIV, INFO )
   22: *
   23: *       .. Scalar Arguments ..
   24: *       CHARACTER          UPLO
   25: *       INTEGER            INFO, N
   26: *       ..
   27: *       .. Array Arguments ..
   28: *       INTEGER            IPIV( * )
   29: *       COMPLEX*16         AP( * )
   30: *       ..
   31: *
   32: *
   33: *> \par Purpose:
   34: *  =============
   35: *>
   36: *> \verbatim
   37: *>
   38: *> ZSPTRF computes the factorization of a complex symmetric matrix A
   39: *> stored in packed format using the Bunch-Kaufman diagonal pivoting
   40: *> method:
   41: *>
   42: *>    A = U*D*U**T  or  A = L*D*L**T
   43: *>
   44: *> where U (or L) is a product of permutation and unit upper (lower)
   45: *> triangular matrices, and D is symmetric and block diagonal with
   46: *> 1-by-1 and 2-by-2 diagonal blocks.
   47: *> \endverbatim
   48: *
   49: *  Arguments:
   50: *  ==========
   51: *
   52: *> \param[in] UPLO
   53: *> \verbatim
   54: *>          UPLO is CHARACTER*1
   55: *>          = 'U':  Upper triangle of A is stored;
   56: *>          = 'L':  Lower triangle of A is stored.
   57: *> \endverbatim
   58: *>
   59: *> \param[in] N
   60: *> \verbatim
   61: *>          N is INTEGER
   62: *>          The order of the matrix A.  N >= 0.
   63: *> \endverbatim
   64: *>
   65: *> \param[in,out] AP
   66: *> \verbatim
   67: *>          AP is COMPLEX*16 array, dimension (N*(N+1)/2)
   68: *>          On entry, the upper or lower triangle of the symmetric matrix
   69: *>          A, packed columnwise in a linear array.  The j-th column of A
   70: *>          is stored in the array AP as follows:
   71: *>          if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j;
   72: *>          if UPLO = 'L', AP(i + (j-1)*(2n-j)/2) = A(i,j) for j<=i<=n.
   73: *>
   74: *>          On exit, the block diagonal matrix D and the multipliers used
   75: *>          to obtain the factor U or L, stored as a packed triangular
   76: *>          matrix overwriting A (see below for further details).
   77: *> \endverbatim
   78: *>
   79: *> \param[out] IPIV
   80: *> \verbatim
   81: *>          IPIV is INTEGER array, dimension (N)
   82: *>          Details of the interchanges and the block structure of D.
   83: *>          If IPIV(k) > 0, then rows and columns k and IPIV(k) were
   84: *>          interchanged and D(k,k) is a 1-by-1 diagonal block.
   85: *>          If UPLO = 'U' and IPIV(k) = IPIV(k-1) < 0, then rows and
   86: *>          columns k-1 and -IPIV(k) were interchanged and D(k-1:k,k-1:k)
   87: *>          is a 2-by-2 diagonal block.  If UPLO = 'L' and IPIV(k) =
   88: *>          IPIV(k+1) < 0, then rows and columns k+1 and -IPIV(k) were
   89: *>          interchanged and D(k:k+1,k:k+1) is a 2-by-2 diagonal block.
   90: *> \endverbatim
   91: *>
   92: *> \param[out] INFO
   93: *> \verbatim
   94: *>          INFO is INTEGER
   95: *>          = 0: successful exit
   96: *>          < 0: if INFO = -i, the i-th argument had an illegal value
   97: *>          > 0: if INFO = i, D(i,i) is exactly zero.  The factorization
   98: *>               has been completed, but the block diagonal matrix D is
   99: *>               exactly singular, and division by zero will occur if it
  100: *>               is used to solve a system of equations.
  101: *> \endverbatim
  102: *
  103: *  Authors:
  104: *  ========
  105: *
  106: *> \author Univ. of Tennessee
  107: *> \author Univ. of California Berkeley
  108: *> \author Univ. of Colorado Denver
  109: *> \author NAG Ltd.
  110: *
  111: *> \ingroup complex16OTHERcomputational
  112: *
  113: *> \par Further Details:
  114: *  =====================
  115: *>
  116: *> \verbatim
  117: *>
  118: *>  5-96 - Based on modifications by J. Lewis, Boeing Computer Services
  119: *>         Company
  120: *>
  121: *>  If UPLO = 'U', then A = U*D*U**T, where
  122: *>     U = P(n)*U(n)* ... *P(k)U(k)* ...,
  123: *>  i.e., U is a product of terms P(k)*U(k), where k decreases from n to
  124: *>  1 in steps of 1 or 2, and D is a block diagonal matrix with 1-by-1
  125: *>  and 2-by-2 diagonal blocks D(k).  P(k) is a permutation matrix as
  126: *>  defined by IPIV(k), and U(k) is a unit upper triangular matrix, such
  127: *>  that if the diagonal block D(k) is of order s (s = 1 or 2), then
  128: *>
  129: *>             (   I    v    0   )   k-s
  130: *>     U(k) =  (   0    I    0   )   s
  131: *>             (   0    0    I   )   n-k
  132: *>                k-s   s   n-k
  133: *>
  134: *>  If s = 1, D(k) overwrites A(k,k), and v overwrites A(1:k-1,k).
  135: *>  If s = 2, the upper triangle of D(k) overwrites A(k-1,k-1), A(k-1,k),
  136: *>  and A(k,k), and v overwrites A(1:k-2,k-1:k).
  137: *>
  138: *>  If UPLO = 'L', then A = L*D*L**T, where
  139: *>     L = P(1)*L(1)* ... *P(k)*L(k)* ...,
  140: *>  i.e., L is a product of terms P(k)*L(k), where k increases from 1 to
  141: *>  n in steps of 1 or 2, and D is a block diagonal matrix with 1-by-1
  142: *>  and 2-by-2 diagonal blocks D(k).  P(k) is a permutation matrix as
  143: *>  defined by IPIV(k), and L(k) is a unit lower triangular matrix, such
  144: *>  that if the diagonal block D(k) is of order s (s = 1 or 2), then
  145: *>
  146: *>             (   I    0     0   )  k-1
  147: *>     L(k) =  (   0    I     0   )  s
  148: *>             (   0    v     I   )  n-k-s+1
  149: *>                k-1   s  n-k-s+1
  150: *>
  151: *>  If s = 1, D(k) overwrites A(k,k), and v overwrites A(k+1:n,k).
  152: *>  If s = 2, the lower triangle of D(k) overwrites A(k,k), A(k+1,k),
  153: *>  and A(k+1,k+1), and v overwrites A(k+2:n,k:k+1).
  154: *> \endverbatim
  155: *>
  156: *  =====================================================================
  157:       SUBROUTINE ZSPTRF( UPLO, N, AP, IPIV, INFO )
  158: *
  159: *  -- LAPACK computational routine --
  160: *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
  161: *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
  162: *
  163: *     .. Scalar Arguments ..
  164:       CHARACTER          UPLO
  165:       INTEGER            INFO, N
  166: *     ..
  167: *     .. Array Arguments ..
  168:       INTEGER            IPIV( * )
  169:       COMPLEX*16         AP( * )
  170: *     ..
  171: *
  172: *  =====================================================================
  173: *
  174: *     .. Parameters ..
  175:       DOUBLE PRECISION   ZERO, ONE
  176:       PARAMETER          ( ZERO = 0.0D+0, ONE = 1.0D+0 )
  177:       DOUBLE PRECISION   EIGHT, SEVTEN
  178:       PARAMETER          ( EIGHT = 8.0D+0, SEVTEN = 17.0D+0 )
  179:       COMPLEX*16         CONE
  180:       PARAMETER          ( CONE = ( 1.0D+0, 0.0D+0 ) )
  181: *     ..
  182: *     .. Local Scalars ..
  183:       LOGICAL            UPPER
  184:       INTEGER            I, IMAX, J, JMAX, K, KC, KK, KNC, KP, KPC,
  185:      $                   KSTEP, KX, NPP
  186:       DOUBLE PRECISION   ABSAKK, ALPHA, COLMAX, ROWMAX
  187:       COMPLEX*16         D11, D12, D21, D22, R1, T, WK, WKM1, WKP1, ZDUM
  188: *     ..
  189: *     .. External Functions ..
  190:       LOGICAL            LSAME
  191:       INTEGER            IZAMAX
  192:       EXTERNAL           LSAME, IZAMAX
  193: *     ..
  194: *     .. External Subroutines ..
  195:       EXTERNAL           XERBLA, ZSCAL, ZSPR, ZSWAP
  196: *     ..
  197: *     .. Intrinsic Functions ..
  198:       INTRINSIC          ABS, DBLE, DIMAG, MAX, SQRT
  199: *     ..
  200: *     .. Statement Functions ..
  201:       DOUBLE PRECISION   CABS1
  202: *     ..
  203: *     .. Statement Function definitions ..
  204:       CABS1( ZDUM ) = ABS( DBLE( ZDUM ) ) + ABS( DIMAG( ZDUM ) )
  205: *     ..
  206: *     .. Executable Statements ..
  207: *
  208: *     Test the input parameters.
  209: *
  210:       INFO = 0
  211:       UPPER = LSAME( UPLO, 'U' )
  212:       IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
  213:          INFO = -1
  214:       ELSE IF( N.LT.0 ) THEN
  215:          INFO = -2
  216:       END IF
  217:       IF( INFO.NE.0 ) THEN
  218:          CALL XERBLA( 'ZSPTRF', -INFO )
  219:          RETURN
  220:       END IF
  221: *
  222: *     Initialize ALPHA for use in choosing pivot block size.
  223: *
  224:       ALPHA = ( ONE+SQRT( SEVTEN ) ) / EIGHT
  225: *
  226:       IF( UPPER ) THEN
  227: *
  228: *        Factorize A as U*D*U**T using the upper triangle of A
  229: *
  230: *        K is the main loop index, decreasing from N to 1 in steps of
  231: *        1 or 2
  232: *
  233:          K = N
  234:          KC = ( N-1 )*N / 2 + 1
  235:    10    CONTINUE
  236:          KNC = KC
  237: *
  238: *        If K < 1, exit from loop
  239: *
  240:          IF( K.LT.1 )
  241:      $      GO TO 110
  242:          KSTEP = 1
  243: *
  244: *        Determine rows and columns to be interchanged and whether
  245: *        a 1-by-1 or 2-by-2 pivot block will be used
  246: *
  247:          ABSAKK = CABS1( AP( KC+K-1 ) )
  248: *
  249: *        IMAX is the row-index of the largest off-diagonal element in
  250: *        column K, and COLMAX is its absolute value
  251: *
  252:          IF( K.GT.1 ) THEN
  253:             IMAX = IZAMAX( K-1, AP( KC ), 1 )
  254:             COLMAX = CABS1( AP( KC+IMAX-1 ) )
  255:          ELSE
  256:             COLMAX = ZERO
  257:          END IF
  258: *
  259:          IF( MAX( ABSAKK, COLMAX ).EQ.ZERO ) THEN
  260: *
  261: *           Column K is zero: set INFO and continue
  262: *
  263:             IF( INFO.EQ.0 )
  264:      $         INFO = K
  265:             KP = K
  266:          ELSE
  267:             IF( ABSAKK.GE.ALPHA*COLMAX ) THEN
  268: *
  269: *              no interchange, use 1-by-1 pivot block
  270: *
  271:                KP = K
  272:             ELSE
  273: *
  274:                ROWMAX = ZERO
  275:                JMAX = IMAX
  276:                KX = IMAX*( IMAX+1 ) / 2 + IMAX
  277:                DO 20 J = IMAX + 1, K
  278:                   IF( CABS1( AP( KX ) ).GT.ROWMAX ) THEN
  279:                      ROWMAX = CABS1( AP( KX ) )
  280:                      JMAX = J
  281:                   END IF
  282:                   KX = KX + J
  283:    20          CONTINUE
  284:                KPC = ( IMAX-1 )*IMAX / 2 + 1
  285:                IF( IMAX.GT.1 ) THEN
  286:                   JMAX = IZAMAX( IMAX-1, AP( KPC ), 1 )
  287:                   ROWMAX = MAX( ROWMAX, CABS1( AP( KPC+JMAX-1 ) ) )
  288:                END IF
  289: *
  290:                IF( ABSAKK.GE.ALPHA*COLMAX*( COLMAX / ROWMAX ) ) THEN
  291: *
  292: *                 no interchange, use 1-by-1 pivot block
  293: *
  294:                   KP = K
  295:                ELSE IF( CABS1( AP( KPC+IMAX-1 ) ).GE.ALPHA*ROWMAX ) THEN
  296: *
  297: *                 interchange rows and columns K and IMAX, use 1-by-1
  298: *                 pivot block
  299: *
  300:                   KP = IMAX
  301:                ELSE
  302: *
  303: *                 interchange rows and columns K-1 and IMAX, use 2-by-2
  304: *                 pivot block
  305: *
  306:                   KP = IMAX
  307:                   KSTEP = 2
  308:                END IF
  309:             END IF
  310: *
  311:             KK = K - KSTEP + 1
  312:             IF( KSTEP.EQ.2 )
  313:      $         KNC = KNC - K + 1
  314:             IF( KP.NE.KK ) THEN
  315: *
  316: *              Interchange rows and columns KK and KP in the leading
  317: *              submatrix A(1:k,1:k)
  318: *
  319:                CALL ZSWAP( KP-1, AP( KNC ), 1, AP( KPC ), 1 )
  320:                KX = KPC + KP - 1
  321:                DO 30 J = KP + 1, KK - 1
  322:                   KX = KX + J - 1
  323:                   T = AP( KNC+J-1 )
  324:                   AP( KNC+J-1 ) = AP( KX )
  325:                   AP( KX ) = T
  326:    30          CONTINUE
  327:                T = AP( KNC+KK-1 )
  328:                AP( KNC+KK-1 ) = AP( KPC+KP-1 )
  329:                AP( KPC+KP-1 ) = T
  330:                IF( KSTEP.EQ.2 ) THEN
  331:                   T = AP( KC+K-2 )
  332:                   AP( KC+K-2 ) = AP( KC+KP-1 )
  333:                   AP( KC+KP-1 ) = T
  334:                END IF
  335:             END IF
  336: *
  337: *           Update the leading submatrix
  338: *
  339:             IF( KSTEP.EQ.1 ) THEN
  340: *
  341: *              1-by-1 pivot block D(k): column k now holds
  342: *
  343: *              W(k) = U(k)*D(k)
  344: *
  345: *              where U(k) is the k-th column of U
  346: *
  347: *              Perform a rank-1 update of A(1:k-1,1:k-1) as
  348: *
  349: *              A := A - U(k)*D(k)*U(k)**T = A - W(k)*1/D(k)*W(k)**T
  350: *
  351:                R1 = CONE / AP( KC+K-1 )
  352:                CALL ZSPR( UPLO, K-1, -R1, AP( KC ), 1, AP )
  353: *
  354: *              Store U(k) in column k
  355: *
  356:                CALL ZSCAL( K-1, R1, AP( KC ), 1 )
  357:             ELSE
  358: *
  359: *              2-by-2 pivot block D(k): columns k and k-1 now hold
  360: *
  361: *              ( W(k-1) W(k) ) = ( U(k-1) U(k) )*D(k)
  362: *
  363: *              where U(k) and U(k-1) are the k-th and (k-1)-th columns
  364: *              of U
  365: *
  366: *              Perform a rank-2 update of A(1:k-2,1:k-2) as
  367: *
  368: *              A := A - ( U(k-1) U(k) )*D(k)*( U(k-1) U(k) )**T
  369: *                 = A - ( W(k-1) W(k) )*inv(D(k))*( W(k-1) W(k) )**T
  370: *
  371:                IF( K.GT.2 ) THEN
  372: *
  373:                   D12 = AP( K-1+( K-1 )*K / 2 )
  374:                   D22 = AP( K-1+( K-2 )*( K-1 ) / 2 ) / D12
  375:                   D11 = AP( K+( K-1 )*K / 2 ) / D12
  376:                   T = CONE / ( D11*D22-CONE )
  377:                   D12 = T / D12
  378: *
  379:                   DO 50 J = K - 2, 1, -1
  380:                      WKM1 = D12*( D11*AP( J+( K-2 )*( K-1 ) / 2 )-
  381:      $                      AP( J+( K-1 )*K / 2 ) )
  382:                      WK = D12*( D22*AP( J+( K-1 )*K / 2 )-
  383:      $                    AP( J+( K-2 )*( K-1 ) / 2 ) )
  384:                      DO 40 I = J, 1, -1
  385:                         AP( I+( J-1 )*J / 2 ) = AP( I+( J-1 )*J / 2 ) -
  386:      $                     AP( I+( K-1 )*K / 2 )*WK -
  387:      $                     AP( I+( K-2 )*( K-1 ) / 2 )*WKM1
  388:    40                CONTINUE
  389:                      AP( J+( K-1 )*K / 2 ) = WK
  390:                      AP( J+( K-2 )*( K-1 ) / 2 ) = WKM1
  391:    50             CONTINUE
  392: *
  393:                END IF
  394:             END IF
  395:          END IF
  396: *
  397: *        Store details of the interchanges in IPIV
  398: *
  399:          IF( KSTEP.EQ.1 ) THEN
  400:             IPIV( K ) = KP
  401:          ELSE
  402:             IPIV( K ) = -KP
  403:             IPIV( K-1 ) = -KP
  404:          END IF
  405: *
  406: *        Decrease K and return to the start of the main loop
  407: *
  408:          K = K - KSTEP
  409:          KC = KNC - K
  410:          GO TO 10
  411: *
  412:       ELSE
  413: *
  414: *        Factorize A as L*D*L**T using the lower triangle of A
  415: *
  416: *        K is the main loop index, increasing from 1 to N in steps of
  417: *        1 or 2
  418: *
  419:          K = 1
  420:          KC = 1
  421:          NPP = N*( N+1 ) / 2
  422:    60    CONTINUE
  423:          KNC = KC
  424: *
  425: *        If K > N, exit from loop
  426: *
  427:          IF( K.GT.N )
  428:      $      GO TO 110
  429:          KSTEP = 1
  430: *
  431: *        Determine rows and columns to be interchanged and whether
  432: *        a 1-by-1 or 2-by-2 pivot block will be used
  433: *
  434:          ABSAKK = CABS1( AP( KC ) )
  435: *
  436: *        IMAX is the row-index of the largest off-diagonal element in
  437: *        column K, and COLMAX is its absolute value
  438: *
  439:          IF( K.LT.N ) THEN
  440:             IMAX = K + IZAMAX( N-K, AP( KC+1 ), 1 )
  441:             COLMAX = CABS1( AP( KC+IMAX-K ) )
  442:          ELSE
  443:             COLMAX = ZERO
  444:          END IF
  445: *
  446:          IF( MAX( ABSAKK, COLMAX ).EQ.ZERO ) THEN
  447: *
  448: *           Column K is zero: set INFO and continue
  449: *
  450:             IF( INFO.EQ.0 )
  451:      $         INFO = K
  452:             KP = K
  453:          ELSE
  454:             IF( ABSAKK.GE.ALPHA*COLMAX ) THEN
  455: *
  456: *              no interchange, use 1-by-1 pivot block
  457: *
  458:                KP = K
  459:             ELSE
  460: *
  461: *              JMAX is the column-index of the largest off-diagonal
  462: *              element in row IMAX, and ROWMAX is its absolute value
  463: *
  464:                ROWMAX = ZERO
  465:                KX = KC + IMAX - K
  466:                DO 70 J = K, IMAX - 1
  467:                   IF( CABS1( AP( KX ) ).GT.ROWMAX ) THEN
  468:                      ROWMAX = CABS1( AP( KX ) )
  469:                      JMAX = J
  470:                   END IF
  471:                   KX = KX + N - J
  472:    70          CONTINUE
  473:                KPC = NPP - ( N-IMAX+1 )*( N-IMAX+2 ) / 2 + 1
  474:                IF( IMAX.LT.N ) THEN
  475:                   JMAX = IMAX + IZAMAX( N-IMAX, AP( KPC+1 ), 1 )
  476:                   ROWMAX = MAX( ROWMAX, CABS1( AP( KPC+JMAX-IMAX ) ) )
  477:                END IF
  478: *
  479:                IF( ABSAKK.GE.ALPHA*COLMAX*( COLMAX / ROWMAX ) ) THEN
  480: *
  481: *                 no interchange, use 1-by-1 pivot block
  482: *
  483:                   KP = K
  484:                ELSE IF( CABS1( AP( KPC ) ).GE.ALPHA*ROWMAX ) THEN
  485: *
  486: *                 interchange rows and columns K and IMAX, use 1-by-1
  487: *                 pivot block
  488: *
  489:                   KP = IMAX
  490:                ELSE
  491: *
  492: *                 interchange rows and columns K+1 and IMAX, use 2-by-2
  493: *                 pivot block
  494: *
  495:                   KP = IMAX
  496:                   KSTEP = 2
  497:                END IF
  498:             END IF
  499: *
  500:             KK = K + KSTEP - 1
  501:             IF( KSTEP.EQ.2 )
  502:      $         KNC = KNC + N - K + 1
  503:             IF( KP.NE.KK ) THEN
  504: *
  505: *              Interchange rows and columns KK and KP in the trailing
  506: *              submatrix A(k:n,k:n)
  507: *
  508:                IF( KP.LT.N )
  509:      $            CALL ZSWAP( N-KP, AP( KNC+KP-KK+1 ), 1, AP( KPC+1 ),
  510:      $                        1 )
  511:                KX = KNC + KP - KK
  512:                DO 80 J = KK + 1, KP - 1
  513:                   KX = KX + N - J + 1
  514:                   T = AP( KNC+J-KK )
  515:                   AP( KNC+J-KK ) = AP( KX )
  516:                   AP( KX ) = T
  517:    80          CONTINUE
  518:                T = AP( KNC )
  519:                AP( KNC ) = AP( KPC )
  520:                AP( KPC ) = T
  521:                IF( KSTEP.EQ.2 ) THEN
  522:                   T = AP( KC+1 )
  523:                   AP( KC+1 ) = AP( KC+KP-K )
  524:                   AP( KC+KP-K ) = T
  525:                END IF
  526:             END IF
  527: *
  528: *           Update the trailing submatrix
  529: *
  530:             IF( KSTEP.EQ.1 ) THEN
  531: *
  532: *              1-by-1 pivot block D(k): column k now holds
  533: *
  534: *              W(k) = L(k)*D(k)
  535: *
  536: *              where L(k) is the k-th column of L
  537: *
  538:                IF( K.LT.N ) THEN
  539: *
  540: *                 Perform a rank-1 update of A(k+1:n,k+1:n) as
  541: *
  542: *                 A := A - L(k)*D(k)*L(k)**T = A - W(k)*(1/D(k))*W(k)**T
  543: *
  544:                   R1 = CONE / AP( KC )
  545:                   CALL ZSPR( UPLO, N-K, -R1, AP( KC+1 ), 1,
  546:      $                       AP( KC+N-K+1 ) )
  547: *
  548: *                 Store L(k) in column K
  549: *
  550:                   CALL ZSCAL( N-K, R1, AP( KC+1 ), 1 )
  551:                END IF
  552:             ELSE
  553: *
  554: *              2-by-2 pivot block D(k): columns K and K+1 now hold
  555: *
  556: *              ( W(k) W(k+1) ) = ( L(k) L(k+1) )*D(k)
  557: *
  558: *              where L(k) and L(k+1) are the k-th and (k+1)-th columns
  559: *              of L
  560: *
  561:                IF( K.LT.N-1 ) THEN
  562: *
  563: *                 Perform a rank-2 update of A(k+2:n,k+2:n) as
  564: *
  565: *                 A := A - ( L(k) L(k+1) )*D(k)*( L(k) L(k+1) )**T
  566: *                    = A - ( W(k) W(k+1) )*inv(D(k))*( W(k) W(k+1) )**T
  567: *
  568: *                 where L(k) and L(k+1) are the k-th and (k+1)-th
  569: *                 columns of L
  570: *
  571:                   D21 = AP( K+1+( K-1 )*( 2*N-K ) / 2 )
  572:                   D11 = AP( K+1+K*( 2*N-K-1 ) / 2 ) / D21
  573:                   D22 = AP( K+( K-1 )*( 2*N-K ) / 2 ) / D21
  574:                   T = CONE / ( D11*D22-CONE )
  575:                   D21 = T / D21
  576: *
  577:                   DO 100 J = K + 2, N
  578:                      WK = D21*( D11*AP( J+( K-1 )*( 2*N-K ) / 2 )-
  579:      $                    AP( J+K*( 2*N-K-1 ) / 2 ) )
  580:                      WKP1 = D21*( D22*AP( J+K*( 2*N-K-1 ) / 2 )-
  581:      $                      AP( J+( K-1 )*( 2*N-K ) / 2 ) )
  582:                      DO 90 I = J, N
  583:                         AP( I+( J-1 )*( 2*N-J ) / 2 ) = AP( I+( J-1 )*
  584:      $                     ( 2*N-J ) / 2 ) - AP( I+( K-1 )*( 2*N-K ) /
  585:      $                     2 )*WK - AP( I+K*( 2*N-K-1 ) / 2 )*WKP1
  586:    90                CONTINUE
  587:                      AP( J+( K-1 )*( 2*N-K ) / 2 ) = WK
  588:                      AP( J+K*( 2*N-K-1 ) / 2 ) = WKP1
  589:   100             CONTINUE
  590:                END IF
  591:             END IF
  592:          END IF
  593: *
  594: *        Store details of the interchanges in IPIV
  595: *
  596:          IF( KSTEP.EQ.1 ) THEN
  597:             IPIV( K ) = KP
  598:          ELSE
  599:             IPIV( K ) = -KP
  600:             IPIV( K+1 ) = -KP
  601:          END IF
  602: *
  603: *        Increase K and return to the start of the main loop
  604: *
  605:          K = K + KSTEP
  606:          KC = KNC + N - K + 2
  607:          GO TO 60
  608: *
  609:       END IF
  610: *
  611:   110 CONTINUE
  612:       RETURN
  613: *
  614: *     End of ZSPTRF
  615: *
  616:       END

CVSweb interface <joel.bertrand@systella.fr>