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CVS tags: rpl-4_1_25, HEAD
Cohérence Lapack.

    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: *> \date November 2011
  112: *
  113: *> \ingroup complex16OTHERcomputational
  114: *
  115: *> \par Further Details:
  116: *  =====================
  117: *>
  118: *> \verbatim
  119: *>
  120: *>  5-96 - Based on modifications by J. Lewis, Boeing Computer Services
  121: *>         Company
  122: *>
  123: *>  If UPLO = 'U', then A = U*D*U**T, where
  124: *>     U = P(n)*U(n)* ... *P(k)U(k)* ...,
  125: *>  i.e., U is a product of terms P(k)*U(k), where k decreases from n to
  126: *>  1 in steps of 1 or 2, and D is a block diagonal matrix with 1-by-1
  127: *>  and 2-by-2 diagonal blocks D(k).  P(k) is a permutation matrix as
  128: *>  defined by IPIV(k), and U(k) is a unit upper triangular matrix, such
  129: *>  that if the diagonal block D(k) is of order s (s = 1 or 2), then
  130: *>
  131: *>             (   I    v    0   )   k-s
  132: *>     U(k) =  (   0    I    0   )   s
  133: *>             (   0    0    I   )   n-k
  134: *>                k-s   s   n-k
  135: *>
  136: *>  If s = 1, D(k) overwrites A(k,k), and v overwrites A(1:k-1,k).
  137: *>  If s = 2, the upper triangle of D(k) overwrites A(k-1,k-1), A(k-1,k),
  138: *>  and A(k,k), and v overwrites A(1:k-2,k-1:k).
  139: *>
  140: *>  If UPLO = 'L', then A = L*D*L**T, where
  141: *>     L = P(1)*L(1)* ... *P(k)*L(k)* ...,
  142: *>  i.e., L is a product of terms P(k)*L(k), where k increases from 1 to
  143: *>  n in steps of 1 or 2, and D is a block diagonal matrix with 1-by-1
  144: *>  and 2-by-2 diagonal blocks D(k).  P(k) is a permutation matrix as
  145: *>  defined by IPIV(k), and L(k) is a unit lower triangular matrix, such
  146: *>  that if the diagonal block D(k) is of order s (s = 1 or 2), then
  147: *>
  148: *>             (   I    0     0   )  k-1
  149: *>     L(k) =  (   0    I     0   )  s
  150: *>             (   0    v     I   )  n-k-s+1
  151: *>                k-1   s  n-k-s+1
  152: *>
  153: *>  If s = 1, D(k) overwrites A(k,k), and v overwrites A(k+1:n,k).
  154: *>  If s = 2, the lower triangle of D(k) overwrites A(k,k), A(k+1,k),
  155: *>  and A(k+1,k+1), and v overwrites A(k+2:n,k:k+1).
  156: *> \endverbatim
  157: *>
  158: *  =====================================================================
  159:       SUBROUTINE ZSPTRF( UPLO, N, AP, IPIV, INFO )
  160: *
  161: *  -- LAPACK computational routine (version 3.4.0) --
  162: *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
  163: *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
  164: *     November 2011
  165: *
  166: *     .. Scalar Arguments ..
  167:       CHARACTER          UPLO
  168:       INTEGER            INFO, N
  169: *     ..
  170: *     .. Array Arguments ..
  171:       INTEGER            IPIV( * )
  172:       COMPLEX*16         AP( * )
  173: *     ..
  174: *
  175: *  =====================================================================
  176: *
  177: *     .. Parameters ..
  178:       DOUBLE PRECISION   ZERO, ONE
  179:       PARAMETER          ( ZERO = 0.0D+0, ONE = 1.0D+0 )
  180:       DOUBLE PRECISION   EIGHT, SEVTEN
  181:       PARAMETER          ( EIGHT = 8.0D+0, SEVTEN = 17.0D+0 )
  182:       COMPLEX*16         CONE
  183:       PARAMETER          ( CONE = ( 1.0D+0, 0.0D+0 ) )
  184: *     ..
  185: *     .. Local Scalars ..
  186:       LOGICAL            UPPER
  187:       INTEGER            I, IMAX, J, JMAX, K, KC, KK, KNC, KP, KPC,
  188:      $                   KSTEP, KX, NPP
  189:       DOUBLE PRECISION   ABSAKK, ALPHA, COLMAX, ROWMAX
  190:       COMPLEX*16         D11, D12, D21, D22, R1, T, WK, WKM1, WKP1, ZDUM
  191: *     ..
  192: *     .. External Functions ..
  193:       LOGICAL            LSAME
  194:       INTEGER            IZAMAX
  195:       EXTERNAL           LSAME, IZAMAX
  196: *     ..
  197: *     .. External Subroutines ..
  198:       EXTERNAL           XERBLA, ZSCAL, ZSPR, ZSWAP
  199: *     ..
  200: *     .. Intrinsic Functions ..
  201:       INTRINSIC          ABS, DBLE, DIMAG, MAX, SQRT
  202: *     ..
  203: *     .. Statement Functions ..
  204:       DOUBLE PRECISION   CABS1
  205: *     ..
  206: *     .. Statement Function definitions ..
  207:       CABS1( ZDUM ) = ABS( DBLE( ZDUM ) ) + ABS( DIMAG( ZDUM ) )
  208: *     ..
  209: *     .. Executable Statements ..
  210: *
  211: *     Test the input parameters.
  212: *
  213:       INFO = 0
  214:       UPPER = LSAME( UPLO, 'U' )
  215:       IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
  216:          INFO = -1
  217:       ELSE IF( N.LT.0 ) THEN
  218:          INFO = -2
  219:       END IF
  220:       IF( INFO.NE.0 ) THEN
  221:          CALL XERBLA( 'ZSPTRF', -INFO )
  222:          RETURN
  223:       END IF
  224: *
  225: *     Initialize ALPHA for use in choosing pivot block size.
  226: *
  227:       ALPHA = ( ONE+SQRT( SEVTEN ) ) / EIGHT
  228: *
  229:       IF( UPPER ) THEN
  230: *
  231: *        Factorize A as U*D*U**T using the upper triangle of A
  232: *
  233: *        K is the main loop index, decreasing from N to 1 in steps of
  234: *        1 or 2
  235: *
  236:          K = N
  237:          KC = ( N-1 )*N / 2 + 1
  238:    10    CONTINUE
  239:          KNC = KC
  240: *
  241: *        If K < 1, exit from loop
  242: *
  243:          IF( K.LT.1 )
  244:      $      GO TO 110
  245:          KSTEP = 1
  246: *
  247: *        Determine rows and columns to be interchanged and whether
  248: *        a 1-by-1 or 2-by-2 pivot block will be used
  249: *
  250:          ABSAKK = CABS1( AP( KC+K-1 ) )
  251: *
  252: *        IMAX is the row-index of the largest off-diagonal element in
  253: *        column K, and COLMAX is its absolute value
  254: *
  255:          IF( K.GT.1 ) THEN
  256:             IMAX = IZAMAX( K-1, AP( KC ), 1 )
  257:             COLMAX = CABS1( AP( KC+IMAX-1 ) )
  258:          ELSE
  259:             COLMAX = ZERO
  260:          END IF
  261: *
  262:          IF( MAX( ABSAKK, COLMAX ).EQ.ZERO ) THEN
  263: *
  264: *           Column K is zero: set INFO and continue
  265: *
  266:             IF( INFO.EQ.0 )
  267:      $         INFO = K
  268:             KP = K
  269:          ELSE
  270:             IF( ABSAKK.GE.ALPHA*COLMAX ) THEN
  271: *
  272: *              no interchange, use 1-by-1 pivot block
  273: *
  274:                KP = K
  275:             ELSE
  276: *
  277:                ROWMAX = ZERO
  278:                JMAX = IMAX
  279:                KX = IMAX*( IMAX+1 ) / 2 + IMAX
  280:                DO 20 J = IMAX + 1, K
  281:                   IF( CABS1( AP( KX ) ).GT.ROWMAX ) THEN
  282:                      ROWMAX = CABS1( AP( KX ) )
  283:                      JMAX = J
  284:                   END IF
  285:                   KX = KX + J
  286:    20          CONTINUE
  287:                KPC = ( IMAX-1 )*IMAX / 2 + 1
  288:                IF( IMAX.GT.1 ) THEN
  289:                   JMAX = IZAMAX( IMAX-1, AP( KPC ), 1 )
  290:                   ROWMAX = MAX( ROWMAX, CABS1( AP( KPC+JMAX-1 ) ) )
  291:                END IF
  292: *
  293:                IF( ABSAKK.GE.ALPHA*COLMAX*( COLMAX / ROWMAX ) ) THEN
  294: *
  295: *                 no interchange, use 1-by-1 pivot block
  296: *
  297:                   KP = K
  298:                ELSE IF( CABS1( AP( KPC+IMAX-1 ) ).GE.ALPHA*ROWMAX ) THEN
  299: *
  300: *                 interchange rows and columns K and IMAX, use 1-by-1
  301: *                 pivot block
  302: *
  303:                   KP = IMAX
  304:                ELSE
  305: *
  306: *                 interchange rows and columns K-1 and IMAX, use 2-by-2
  307: *                 pivot block
  308: *
  309:                   KP = IMAX
  310:                   KSTEP = 2
  311:                END IF
  312:             END IF
  313: *
  314:             KK = K - KSTEP + 1
  315:             IF( KSTEP.EQ.2 )
  316:      $         KNC = KNC - K + 1
  317:             IF( KP.NE.KK ) THEN
  318: *
  319: *              Interchange rows and columns KK and KP in the leading
  320: *              submatrix A(1:k,1:k)
  321: *
  322:                CALL ZSWAP( KP-1, AP( KNC ), 1, AP( KPC ), 1 )
  323:                KX = KPC + KP - 1
  324:                DO 30 J = KP + 1, KK - 1
  325:                   KX = KX + J - 1
  326:                   T = AP( KNC+J-1 )
  327:                   AP( KNC+J-1 ) = AP( KX )
  328:                   AP( KX ) = T
  329:    30          CONTINUE
  330:                T = AP( KNC+KK-1 )
  331:                AP( KNC+KK-1 ) = AP( KPC+KP-1 )
  332:                AP( KPC+KP-1 ) = T
  333:                IF( KSTEP.EQ.2 ) THEN
  334:                   T = AP( KC+K-2 )
  335:                   AP( KC+K-2 ) = AP( KC+KP-1 )
  336:                   AP( KC+KP-1 ) = T
  337:                END IF
  338:             END IF
  339: *
  340: *           Update the leading submatrix
  341: *
  342:             IF( KSTEP.EQ.1 ) THEN
  343: *
  344: *              1-by-1 pivot block D(k): column k now holds
  345: *
  346: *              W(k) = U(k)*D(k)
  347: *
  348: *              where U(k) is the k-th column of U
  349: *
  350: *              Perform a rank-1 update of A(1:k-1,1:k-1) as
  351: *
  352: *              A := A - U(k)*D(k)*U(k)**T = A - W(k)*1/D(k)*W(k)**T
  353: *
  354:                R1 = CONE / AP( KC+K-1 )
  355:                CALL ZSPR( UPLO, K-1, -R1, AP( KC ), 1, AP )
  356: *
  357: *              Store U(k) in column k
  358: *
  359:                CALL ZSCAL( K-1, R1, AP( KC ), 1 )
  360:             ELSE
  361: *
  362: *              2-by-2 pivot block D(k): columns k and k-1 now hold
  363: *
  364: *              ( W(k-1) W(k) ) = ( U(k-1) U(k) )*D(k)
  365: *
  366: *              where U(k) and U(k-1) are the k-th and (k-1)-th columns
  367: *              of U
  368: *
  369: *              Perform a rank-2 update of A(1:k-2,1:k-2) as
  370: *
  371: *              A := A - ( U(k-1) U(k) )*D(k)*( U(k-1) U(k) )**T
  372: *                 = A - ( W(k-1) W(k) )*inv(D(k))*( W(k-1) W(k) )**T
  373: *
  374:                IF( K.GT.2 ) THEN
  375: *
  376:                   D12 = AP( K-1+( K-1 )*K / 2 )
  377:                   D22 = AP( K-1+( K-2 )*( K-1 ) / 2 ) / D12
  378:                   D11 = AP( K+( K-1 )*K / 2 ) / D12
  379:                   T = CONE / ( D11*D22-CONE )
  380:                   D12 = T / D12
  381: *
  382:                   DO 50 J = K - 2, 1, -1
  383:                      WKM1 = D12*( D11*AP( J+( K-2 )*( K-1 ) / 2 )-
  384:      $                      AP( J+( K-1 )*K / 2 ) )
  385:                      WK = D12*( D22*AP( J+( K-1 )*K / 2 )-
  386:      $                    AP( J+( K-2 )*( K-1 ) / 2 ) )
  387:                      DO 40 I = J, 1, -1
  388:                         AP( I+( J-1 )*J / 2 ) = AP( I+( J-1 )*J / 2 ) -
  389:      $                     AP( I+( K-1 )*K / 2 )*WK -
  390:      $                     AP( I+( K-2 )*( K-1 ) / 2 )*WKM1
  391:    40                CONTINUE
  392:                      AP( J+( K-1 )*K / 2 ) = WK
  393:                      AP( J+( K-2 )*( K-1 ) / 2 ) = WKM1
  394:    50             CONTINUE
  395: *
  396:                END IF
  397:             END IF
  398:          END IF
  399: *
  400: *        Store details of the interchanges in IPIV
  401: *
  402:          IF( KSTEP.EQ.1 ) THEN
  403:             IPIV( K ) = KP
  404:          ELSE
  405:             IPIV( K ) = -KP
  406:             IPIV( K-1 ) = -KP
  407:          END IF
  408: *
  409: *        Decrease K and return to the start of the main loop
  410: *
  411:          K = K - KSTEP
  412:          KC = KNC - K
  413:          GO TO 10
  414: *
  415:       ELSE
  416: *
  417: *        Factorize A as L*D*L**T using the lower triangle of A
  418: *
  419: *        K is the main loop index, increasing from 1 to N in steps of
  420: *        1 or 2
  421: *
  422:          K = 1
  423:          KC = 1
  424:          NPP = N*( N+1 ) / 2
  425:    60    CONTINUE
  426:          KNC = KC
  427: *
  428: *        If K > N, exit from loop
  429: *
  430:          IF( K.GT.N )
  431:      $      GO TO 110
  432:          KSTEP = 1
  433: *
  434: *        Determine rows and columns to be interchanged and whether
  435: *        a 1-by-1 or 2-by-2 pivot block will be used
  436: *
  437:          ABSAKK = CABS1( AP( KC ) )
  438: *
  439: *        IMAX is the row-index of the largest off-diagonal element in
  440: *        column K, and COLMAX is its absolute value
  441: *
  442:          IF( K.LT.N ) THEN
  443:             IMAX = K + IZAMAX( N-K, AP( KC+1 ), 1 )
  444:             COLMAX = CABS1( AP( KC+IMAX-K ) )
  445:          ELSE
  446:             COLMAX = ZERO
  447:          END IF
  448: *
  449:          IF( MAX( ABSAKK, COLMAX ).EQ.ZERO ) THEN
  450: *
  451: *           Column K is zero: set INFO and continue
  452: *
  453:             IF( INFO.EQ.0 )
  454:      $         INFO = K
  455:             KP = K
  456:          ELSE
  457:             IF( ABSAKK.GE.ALPHA*COLMAX ) THEN
  458: *
  459: *              no interchange, use 1-by-1 pivot block
  460: *
  461:                KP = K
  462:             ELSE
  463: *
  464: *              JMAX is the column-index of the largest off-diagonal
  465: *              element in row IMAX, and ROWMAX is its absolute value
  466: *
  467:                ROWMAX = ZERO
  468:                KX = KC + IMAX - K
  469:                DO 70 J = K, IMAX - 1
  470:                   IF( CABS1( AP( KX ) ).GT.ROWMAX ) THEN
  471:                      ROWMAX = CABS1( AP( KX ) )
  472:                      JMAX = J
  473:                   END IF
  474:                   KX = KX + N - J
  475:    70          CONTINUE
  476:                KPC = NPP - ( N-IMAX+1 )*( N-IMAX+2 ) / 2 + 1
  477:                IF( IMAX.LT.N ) THEN
  478:                   JMAX = IMAX + IZAMAX( N-IMAX, AP( KPC+1 ), 1 )
  479:                   ROWMAX = MAX( ROWMAX, CABS1( AP( KPC+JMAX-IMAX ) ) )
  480:                END IF
  481: *
  482:                IF( ABSAKK.GE.ALPHA*COLMAX*( COLMAX / ROWMAX ) ) THEN
  483: *
  484: *                 no interchange, use 1-by-1 pivot block
  485: *
  486:                   KP = K
  487:                ELSE IF( CABS1( AP( KPC ) ).GE.ALPHA*ROWMAX ) THEN
  488: *
  489: *                 interchange rows and columns K and IMAX, use 1-by-1
  490: *                 pivot block
  491: *
  492:                   KP = IMAX
  493:                ELSE
  494: *
  495: *                 interchange rows and columns K+1 and IMAX, use 2-by-2
  496: *                 pivot block
  497: *
  498:                   KP = IMAX
  499:                   KSTEP = 2
  500:                END IF
  501:             END IF
  502: *
  503:             KK = K + KSTEP - 1
  504:             IF( KSTEP.EQ.2 )
  505:      $         KNC = KNC + N - K + 1
  506:             IF( KP.NE.KK ) THEN
  507: *
  508: *              Interchange rows and columns KK and KP in the trailing
  509: *              submatrix A(k:n,k:n)
  510: *
  511:                IF( KP.LT.N )
  512:      $            CALL ZSWAP( N-KP, AP( KNC+KP-KK+1 ), 1, AP( KPC+1 ),
  513:      $                        1 )
  514:                KX = KNC + KP - KK
  515:                DO 80 J = KK + 1, KP - 1
  516:                   KX = KX + N - J + 1
  517:                   T = AP( KNC+J-KK )
  518:                   AP( KNC+J-KK ) = AP( KX )
  519:                   AP( KX ) = T
  520:    80          CONTINUE
  521:                T = AP( KNC )
  522:                AP( KNC ) = AP( KPC )
  523:                AP( KPC ) = T
  524:                IF( KSTEP.EQ.2 ) THEN
  525:                   T = AP( KC+1 )
  526:                   AP( KC+1 ) = AP( KC+KP-K )
  527:                   AP( KC+KP-K ) = T
  528:                END IF
  529:             END IF
  530: *
  531: *           Update the trailing submatrix
  532: *
  533:             IF( KSTEP.EQ.1 ) THEN
  534: *
  535: *              1-by-1 pivot block D(k): column k now holds
  536: *
  537: *              W(k) = L(k)*D(k)
  538: *
  539: *              where L(k) is the k-th column of L
  540: *
  541:                IF( K.LT.N ) THEN
  542: *
  543: *                 Perform a rank-1 update of A(k+1:n,k+1:n) as
  544: *
  545: *                 A := A - L(k)*D(k)*L(k)**T = A - W(k)*(1/D(k))*W(k)**T
  546: *
  547:                   R1 = CONE / AP( KC )
  548:                   CALL ZSPR( UPLO, N-K, -R1, AP( KC+1 ), 1,
  549:      $                       AP( KC+N-K+1 ) )
  550: *
  551: *                 Store L(k) in column K
  552: *
  553:                   CALL ZSCAL( N-K, R1, AP( KC+1 ), 1 )
  554:                END IF
  555:             ELSE
  556: *
  557: *              2-by-2 pivot block D(k): columns K and K+1 now hold
  558: *
  559: *              ( W(k) W(k+1) ) = ( L(k) L(k+1) )*D(k)
  560: *
  561: *              where L(k) and L(k+1) are the k-th and (k+1)-th columns
  562: *              of L
  563: *
  564:                IF( K.LT.N-1 ) THEN
  565: *
  566: *                 Perform a rank-2 update of A(k+2:n,k+2:n) as
  567: *
  568: *                 A := A - ( L(k) L(k+1) )*D(k)*( L(k) L(k+1) )**T
  569: *                    = A - ( W(k) W(k+1) )*inv(D(k))*( W(k) W(k+1) )**T
  570: *
  571: *                 where L(k) and L(k+1) are the k-th and (k+1)-th
  572: *                 columns of L
  573: *
  574:                   D21 = AP( K+1+( K-1 )*( 2*N-K ) / 2 )
  575:                   D11 = AP( K+1+K*( 2*N-K-1 ) / 2 ) / D21
  576:                   D22 = AP( K+( K-1 )*( 2*N-K ) / 2 ) / D21
  577:                   T = CONE / ( D11*D22-CONE )
  578:                   D21 = T / D21
  579: *
  580:                   DO 100 J = K + 2, N
  581:                      WK = D21*( D11*AP( J+( K-1 )*( 2*N-K ) / 2 )-
  582:      $                    AP( J+K*( 2*N-K-1 ) / 2 ) )
  583:                      WKP1 = D21*( D22*AP( J+K*( 2*N-K-1 ) / 2 )-
  584:      $                      AP( J+( K-1 )*( 2*N-K ) / 2 ) )
  585:                      DO 90 I = J, N
  586:                         AP( I+( J-1 )*( 2*N-J ) / 2 ) = AP( I+( J-1 )*
  587:      $                     ( 2*N-J ) / 2 ) - AP( I+( K-1 )*( 2*N-K ) /
  588:      $                     2 )*WK - AP( I+K*( 2*N-K-1 ) / 2 )*WKP1
  589:    90                CONTINUE
  590:                      AP( J+( K-1 )*( 2*N-K ) / 2 ) = WK
  591:                      AP( J+K*( 2*N-K-1 ) / 2 ) = WKP1
  592:   100             CONTINUE
  593:                END IF
  594:             END IF
  595:          END IF
  596: *
  597: *        Store details of the interchanges in IPIV
  598: *
  599:          IF( KSTEP.EQ.1 ) THEN
  600:             IPIV( K ) = KP
  601:          ELSE
  602:             IPIV( K ) = -KP
  603:             IPIV( K+1 ) = -KP
  604:          END IF
  605: *
  606: *        Increase K and return to the start of the main loop
  607: *
  608:          K = K + KSTEP
  609:          KC = KNC + N - K + 2
  610:          GO TO 60
  611: *
  612:       END IF
  613: *
  614:   110 CONTINUE
  615:       RETURN
  616: *
  617: *     End of ZSPTRF
  618: *
  619:       END

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