          SUBROUTINE CSPTRF( UPLO, N, AP, IPIV, INFO )
    *
    *  -- LAPACK routine (version 3.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
    *     November 2006
    *
    *     .. Scalar Arguments ..
          CHARACTER          UPLO
          INTEGER            INFO, N
    *     ..
    *     .. Array Arguments ..
          INTEGER            IPIV( * )
          COMPLEX            AP( * )
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  CSPTRF computes the factorization of a complex symmetric matrix A
    *  stored in packed format using the Bunch-Kaufman diagonal pivoting
    *  method:
    *
    *     A = U*D*U**T  or  A = L*D*L**T
    *
    *  where U (or L) is a product of permutation and unit upper (lower)
    *  triangular matrices, and D is symmetric and block diagonal with
    *  1-by-1 and 2-by-2 diagonal blocks.
    *
    *  Arguments
    *  =========
    *
    *  UPLO    (input) CHARACTER*1
    *          = 'U':  Upper triangle of A is stored;
    *          = 'L':  Lower triangle of A is stored.
    *
    *  N       (input) INTEGER
    *          The order of the matrix A.  N >= 0.
    *
    *  AP      (input/output) COMPLEX array, dimension (N*(N+1)/2)
    *          On entry, the upper or lower triangle of the symmetric matrix
    *          A, packed columnwise in a linear array.  The j-th column of A
    *          is stored in the array AP as follows:
    *          if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j;
    *          if UPLO = 'L', AP(i + (j-1)*(2n-j)/2) = A(i,j) for j<=i<=n.
    *
    *          On exit, the block diagonal matrix D and the multipliers used
    *          to obtain the factor U or L, stored as a packed triangular
    *          matrix overwriting A (see below for further details).
    *
    *  IPIV    (output) INTEGER array, dimension (N)
    *          Details of the interchanges and the block structure of D.
    *          If IPIV(k) > 0, then rows and columns k and IPIV(k) were
    *          interchanged and D(k,k) is a 1-by-1 diagonal block.
    *          If UPLO = 'U' and IPIV(k) = IPIV(k-1) < 0, then rows and
    *          columns k-1 and -IPIV(k) were interchanged and D(k-1:k,k-1:k)
    *          is a 2-by-2 diagonal block.  If UPLO = 'L' and IPIV(k) =
    *          IPIV(k+1) < 0, then rows and columns k+1 and -IPIV(k) were
    *          interchanged and D(k:k+1,k:k+1) is a 2-by-2 diagonal block.
    *
    *  INFO    (output) INTEGER
    *          = 0: successful exit
    *          < 0: if INFO = -i, the i-th argument had an illegal value
    *          > 0: if INFO = i, D(i,i) is exactly zero.  The factorization
    *               has been completed, but the block diagonal matrix D is
    *               exactly singular, and division by zero will occur if it
    *               is used to solve a system of equations.
    *
    *  Further Details
    *  ===============
    *
    *  5-96 - Based on modifications by J. Lewis, Boeing Computer Services
    *         Company
    *
    *  If UPLO = 'U', then A = U*D*U', where
    *     U = P(n)*U(n)* ... *P(k)U(k)* ...,
    *  i.e., U is a product of terms P(k)*U(k), where k decreases from n to
    *  1 in steps of 1 or 2, and D is a block diagonal matrix with 1-by-1
    *  and 2-by-2 diagonal blocks D(k).  P(k) is a permutation matrix as
    *  defined by IPIV(k), and U(k) is a unit upper triangular matrix, such
    *  that if the diagonal block D(k) is of order s (s = 1 or 2), then
    *
    *             (   I    v    0   )   k-s
    *     U(k) =  (   0    I    0   )   s
    *             (   0    0    I   )   n-k
    *                k-s   s   n-k
    *
    *  If s = 1, D(k) overwrites A(k,k), and v overwrites A(1:k-1,k).
    *  If s = 2, the upper triangle of D(k) overwrites A(k-1,k-1), A(k-1,k),
    *  and A(k,k), and v overwrites A(1:k-2,k-1:k).
    *
    *  If UPLO = 'L', then A = L*D*L', where
    *     L = P(1)*L(1)* ... *P(k)*L(k)* ...,
    *  i.e., L is a product of terms P(k)*L(k), where k increases from 1 to
    *  n in steps of 1 or 2, and D is a block diagonal matrix with 1-by-1
    *  and 2-by-2 diagonal blocks D(k).  P(k) is a permutation matrix as
    *  defined by IPIV(k), and L(k) is a unit lower triangular matrix, such
    *  that if the diagonal block D(k) is of order s (s = 1 or 2), then
    *
    *             (   I    0     0   )  k-1
    *     L(k) =  (   0    I     0   )  s
    *             (   0    v     I   )  n-k-s+1
    *                k-1   s  n-k-s+1
    *
    *  If s = 1, D(k) overwrites A(k,k), and v overwrites A(k+1:n,k).
    *  If s = 2, the lower triangle of D(k) overwrites A(k,k), A(k+1,k),
    *  and A(k+1,k+1), and v overwrites A(k+2:n,k:k+1).
    *
    *  =====================================================================
    *
    *     .. Parameters ..
          REAL               ZERO, ONE
          PARAMETER          ( ZERO = 0.0E+0, ONE = 1.0E+0 )
          REAL               EIGHT, SEVTEN
          PARAMETER          ( EIGHT = 8.0E+0, SEVTEN = 17.0E+0 )
          COMPLEX            CONE
          PARAMETER          ( CONE = ( 1.0E+0, 0.0E+0 ) )
    *     ..
    *     .. Local Scalars ..
          LOGICAL            UPPER
          INTEGER            I, IMAX, J, JMAX, K, KC, KK, KNC, KP, KPC,
         $                   KSTEP, KX, NPP
          REAL               ABSAKK, ALPHA, COLMAX, ROWMAX
          COMPLEX            D11, D12, D21, D22, R1, T, WK, WKM1, WKP1, ZDUM
    *     ..
    *     .. External Functions ..
          LOGICAL            LSAME
          INTEGER            ICAMAX
          EXTERNAL           LSAME, ICAMAX
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           CSCAL, CSPR, CSWAP, XERBLA
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          ABS, AIMAG, MAX, REAL, SQRT
    *     ..
    *     .. Statement Functions ..
          REAL               CABS1
    *     ..
    *     .. Statement Function definitions ..
          CABS1( ZDUM ) = ABS( REAL( ZDUM ) ) + ABS( AIMAG( ZDUM ) )
    *     ..
    *     .. Executable Statements ..
    *
    *     Test the input parameters.
    *
          INFO = 0
          UPPER = LSAME( UPLO, 'U' )
          IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
             INFO = -1
          ELSE IF( N.LT.0 ) THEN
             INFO = -2
          END IF
          IF( INFO.NE.0 ) THEN
             CALL XERBLA( 'CSPTRF', -INFO )
             RETURN
          END IF
    *
    *     Initialize ALPHA for use in choosing pivot block size.
    *
          ALPHA = ( ONE+SQRT( SEVTEN ) ) / EIGHT
    *
          IF( UPPER ) THEN
    *
    *        Factorize A as U*D*U' using the upper triangle of A
    *
    *        K is the main loop index, decreasing from N to 1 in steps of
    *        1 or 2
    *
             K = N
             KC = ( N-1 )*N / 2 + 1
       10    CONTINUE
             KNC = KC
    *
    *        If K < 1, exit from loop
    *
             IF( K.LT.1 )
         $      GO TO 110
             KSTEP = 1
    *
    *        Determine rows and columns to be interchanged and whether
    *        a 1-by-1 or 2-by-2 pivot block will be used
    *
             ABSAKK = CABS1( AP( KC+K-1 ) )
    *
    *        IMAX is the row-index of the largest off-diagonal element in
    *        column K, and COLMAX is its absolute value
    *
             IF( K.GT.1 ) THEN
                IMAX = ICAMAX( K-1, AP( KC ), 1 )
                COLMAX = CABS1( AP( KC+IMAX-1 ) )
             ELSE
                COLMAX = ZERO
             END IF
    *
             IF( MAX( ABSAKK, COLMAX ).EQ.ZERO ) THEN
    *
    *           Column K is zero: set INFO and continue
    *
                IF( INFO.EQ.0 )
         $         INFO = K
                KP = K
             ELSE
                IF( ABSAKK.GE.ALPHA*COLMAX ) THEN
    *
    *              no interchange, use 1-by-1 pivot block
    *
                   KP = K
                ELSE
    *
    *              JMAX is the column-index of the largest off-diagonal
    *              element in row IMAX, and ROWMAX is its absolute value
    *
                   ROWMAX = ZERO
                   JMAX = IMAX
                   KX = IMAX*( IMAX+1 ) / 2 + IMAX
                   DO 20 J = IMAX + 1, K
                      IF( CABS1( AP( KX ) ).GT.ROWMAX ) THEN
                         ROWMAX = CABS1( AP( KX ) )
                         JMAX = J
                      END IF
                      KX = KX + J
       20          CONTINUE
                   KPC = ( IMAX-1 )*IMAX / 2 + 1
                   IF( IMAX.GT.1 ) THEN
                      JMAX = ICAMAX( IMAX-1, AP( KPC ), 1 )
                      ROWMAX = MAX( ROWMAX, CABS1( AP( KPC+JMAX-1 ) ) )
                   END IF
    *
                   IF( ABSAKK.GE.ALPHA*COLMAX*( COLMAX / ROWMAX ) ) THEN
    *
    *                 no interchange, use 1-by-1 pivot block
    *
                      KP = K
                   ELSE IF( CABS1( AP( KPC+IMAX-1 ) ).GE.ALPHA*ROWMAX ) THEN
    *
    *                 interchange rows and columns K and IMAX, use 1-by-1
    *                 pivot block
    *
                      KP = IMAX
                   ELSE
    *
    *                 interchange rows and columns K-1 and IMAX, use 2-by-2
    *                 pivot block
    *
                      KP = IMAX
                      KSTEP = 2
                   END IF
                END IF
    *
                KK = K - KSTEP + 1
                IF( KSTEP.EQ.2 )
         $         KNC = KNC - K + 1
                IF( KP.NE.KK ) THEN
    *
    *              Interchange rows and columns KK and KP in the leading
    *              submatrix A(1:k,1:k)
    *
                   CALL CSWAP( KP-1, AP( KNC ), 1, AP( KPC ), 1 )
                   KX = KPC + KP - 1
                   DO 30 J = KP + 1, KK - 1
                      KX = KX + J - 1
                      T = AP( KNC+J-1 )
                      AP( KNC+J-1 ) = AP( KX )
                      AP( KX ) = T
       30          CONTINUE
                   T = AP( KNC+KK-1 )
                   AP( KNC+KK-1 ) = AP( KPC+KP-1 )
                   AP( KPC+KP-1 ) = T
                   IF( KSTEP.EQ.2 ) THEN
                      T = AP( KC+K-2 )
                      AP( KC+K-2 ) = AP( KC+KP-1 )
                      AP( KC+KP-1 ) = T
                   END IF
                END IF
    *
    *           Update the leading submatrix
    *
                IF( KSTEP.EQ.1 ) THEN
    *
    *              1-by-1 pivot block D(k): column k now holds
    *
    *              W(k) = U(k)*D(k)
    *
    *              where U(k) is the k-th column of U
    *
    *              Perform a rank-1 update of A(1:k-1,1:k-1) as
    *
    *              A := A - U(k)*D(k)*U(k)' = A - W(k)*1/D(k)*W(k)'
    *
                   R1 = CONE / AP( KC+K-1 )
                   CALL CSPR( UPLO, K-1, -R1, AP( KC ), 1, AP )
    *
    *              Store U(k) in column k
    *
                   CALL CSCAL( K-1, R1, AP( KC ), 1 )
                ELSE
    *
    *              2-by-2 pivot block D(k): columns k and k-1 now hold
    *
    *              ( W(k-1) W(k) ) = ( U(k-1) U(k) )*D(k)
    *
    *              where U(k) and U(k-1) are the k-th and (k-1)-th columns
    *              of U
    *
    *              Perform a rank-2 update of A(1:k-2,1:k-2) as
    *
    *              A := A - ( U(k-1) U(k) )*D(k)*( U(k-1) U(k) )'
    *                 = A - ( W(k-1) W(k) )*inv(D(k))*( W(k-1) W(k) )'
    *
                   IF( K.GT.2 ) THEN
    *
                      D12 = AP( K-1+( K-1 )*K / 2 )
                      D22 = AP( K-1+( K-2 )*( K-1 ) / 2 ) / D12
                      D11 = AP( K+( K-1 )*K / 2 ) / D12
                      T = CONE / ( D11*D22-CONE )
                      D12 = T / D12
    *
                      DO 50 J = K - 2, 1, -1
                         WKM1 = D12*( D11*AP( J+( K-2 )*( K-1 ) / 2 )-
         $                      AP( J+( K-1 )*K / 2 ) )
                         WK = D12*( D22*AP( J+( K-1 )*K / 2 )-
         $                    AP( J+( K-2 )*( K-1 ) / 2 ) )
                         DO 40 I = J, 1, -1
                            AP( I+( J-1 )*J / 2 ) = AP( I+( J-1 )*J / 2 ) -
         $                     AP( I+( K-1 )*K / 2 )*WK -
         $                     AP( I+( K-2 )*( K-1 ) / 2 )*WKM1
       40                CONTINUE
                         AP( J+( K-1 )*K / 2 ) = WK
                         AP( J+( K-2 )*( K-1 ) / 2 ) = WKM1
       50             CONTINUE
    *
                   END IF
                END IF
             END IF
    *
    *        Store details of the interchanges in IPIV
    *
             IF( KSTEP.EQ.1 ) THEN
                IPIV( K ) = KP
             ELSE
                IPIV( K ) = -KP
                IPIV( K-1 ) = -KP
             END IF
    *
    *        Decrease K and return to the start of the main loop
    *
             K = K - KSTEP
             KC = KNC - K
             GO TO 10
    *
          ELSE
    *
    *        Factorize A as L*D*L' using the lower triangle of A
    *
    *        K is the main loop index, increasing from 1 to N in steps of
    *        1 or 2
    *
             K = 1
             KC = 1
             NPP = N*( N+1 ) / 2
       60    CONTINUE
             KNC = KC
    *
    *        If K > N, exit from loop
    *
             IF( K.GT.N )
         $      GO TO 110
             KSTEP = 1
    *
    *        Determine rows and columns to be interchanged and whether
    *        a 1-by-1 or 2-by-2 pivot block will be used
    *
             ABSAKK = CABS1( AP( KC ) )
    *
    *        IMAX is the row-index of the largest off-diagonal element in
    *        column K, and COLMAX is its absolute value
    *
             IF( K.LT.N ) THEN
                IMAX = K + ICAMAX( N-K, AP( KC+1 ), 1 )
                COLMAX = CABS1( AP( KC+IMAX-K ) )
             ELSE
                COLMAX = ZERO
             END IF
    *
             IF( MAX( ABSAKK, COLMAX ).EQ.ZERO ) THEN
    *
    *           Column K is zero: set INFO and continue
    *
                IF( INFO.EQ.0 )
         $         INFO = K
                KP = K
             ELSE
                IF( ABSAKK.GE.ALPHA*COLMAX ) THEN
    *
    *              no interchange, use 1-by-1 pivot block
    *
                   KP = K
                ELSE
    *
    *              JMAX is the column-index of the largest off-diagonal
    *              element in row IMAX, and ROWMAX is its absolute value
    *
                   ROWMAX = ZERO
                   KX = KC + IMAX - K
                   DO 70 J = K, IMAX - 1
                      IF( CABS1( AP( KX ) ).GT.ROWMAX ) THEN
                         ROWMAX = CABS1( AP( KX ) )
                         JMAX = J
                      END IF
                      KX = KX + N - J
       70          CONTINUE
                   KPC = NPP - ( N-IMAX+1 )*( N-IMAX+2 ) / 2 + 1
                   IF( IMAX.LT.N ) THEN
                      JMAX = IMAX + ICAMAX( N-IMAX, AP( KPC+1 ), 1 )
                      ROWMAX = MAX( ROWMAX, CABS1( AP( KPC+JMAX-IMAX ) ) )
                   END IF
    *
                   IF( ABSAKK.GE.ALPHA*COLMAX*( COLMAX / ROWMAX ) ) THEN
    *
    *                 no interchange, use 1-by-1 pivot block
    *
                      KP = K
                   ELSE IF( CABS1( AP( KPC ) ).GE.ALPHA*ROWMAX ) THEN
    *
    *                 interchange rows and columns K and IMAX, use 1-by-1
    *                 pivot block
    *
                      KP = IMAX
                   ELSE
    *
    *                 interchange rows and columns K+1 and IMAX, use 2-by-2
    *                 pivot block
    *
                      KP = IMAX
                      KSTEP = 2
                   END IF
                END IF
    *
                KK = K + KSTEP - 1
                IF( KSTEP.EQ.2 )
         $         KNC = KNC + N - K + 1
                IF( KP.NE.KK ) THEN
    *
    *              Interchange rows and columns KK and KP in the trailing
    *              submatrix A(k:n,k:n)
    *
                   IF( KP.LT.N )
         $            CALL CSWAP( N-KP, AP( KNC+KP-KK+1 ), 1, AP( KPC+1 ),
         $                        1 )
                   KX = KNC + KP - KK
                   DO 80 J = KK + 1, KP - 1
                      KX = KX + N - J + 1
                      T = AP( KNC+J-KK )
                      AP( KNC+J-KK ) = AP( KX )
                      AP( KX ) = T
       80          CONTINUE
                   T = AP( KNC )
                   AP( KNC ) = AP( KPC )
                   AP( KPC ) = T
                   IF( KSTEP.EQ.2 ) THEN
                      T = AP( KC+1 )
                      AP( KC+1 ) = AP( KC+KP-K )
                      AP( KC+KP-K ) = T
                   END IF
                END IF
    *
    *           Update the trailing submatrix
    *
                IF( KSTEP.EQ.1 ) THEN
    *
    *              1-by-1 pivot block D(k): column k now holds
    *
    *              W(k) = L(k)*D(k)
    *
    *              where L(k) is the k-th column of L
    *
                   IF( K.LT.N ) THEN
    *
    *                 Perform a rank-1 update of A(k+1:n,k+1:n) as
    *
    *                 A := A - L(k)*D(k)*L(k)' = A - W(k)*(1/D(k))*W(k)'
    *
                      R1 = CONE / AP( KC )
                      CALL CSPR( UPLO, N-K, -R1, AP( KC+1 ), 1,
         $                       AP( KC+N-K+1 ) )
    *
    *                 Store L(k) in column K
    *
                      CALL CSCAL( N-K, R1, AP( KC+1 ), 1 )
                   END IF
                ELSE
    *
    *              2-by-2 pivot block D(k): columns K and K+1 now hold
    *
    *              ( W(k) W(k+1) ) = ( L(k) L(k+1) )*D(k)
    *
    *              where L(k) and L(k+1) are the k-th and (k+1)-th columns
    *              of L
    *
                   IF( K.LT.N-1 ) THEN
    *
    *                 Perform a rank-2 update of A(k+2:n,k+2:n) as
    *
    *                 A := A - ( L(k) L(k+1) )*D(k)*( L(k) L(k+1) )'
    *                    = A - ( W(k) W(k+1) )*inv(D(k))*( W(k) W(k+1) )'
    *
    *                 where L(k) and L(k+1) are the k-th and (k+1)-th
    *                 columns of L
    *
                      D21 = AP( K+1+( K-1 )*( 2*N-K ) / 2 )
                      D11 = AP( K+1+K*( 2*N-K-1 ) / 2 ) / D21
                      D22 = AP( K+( K-1 )*( 2*N-K ) / 2 ) / D21
                      T = CONE / ( D11*D22-CONE )
                      D21 = T / D21
    *
                      DO 100 J = K + 2, N
                         WK = D21*( D11*AP( J+( K-1 )*( 2*N-K ) / 2 )-
         $                    AP( J+K*( 2*N-K-1 ) / 2 ) )
                         WKP1 = D21*( D22*AP( J+K*( 2*N-K-1 ) / 2 )-
         $                      AP( J+( K-1 )*( 2*N-K ) / 2 ) )
                         DO 90 I = J, N
                            AP( I+( J-1 )*( 2*N-J ) / 2 ) = AP( I+( J-1 )*
         $                     ( 2*N-J ) / 2 ) - AP( I+( K-1 )*( 2*N-K ) /
         $                     2 )*WK - AP( I+K*( 2*N-K-1 ) / 2 )*WKP1
       90                CONTINUE
                         AP( J+( K-1 )*( 2*N-K ) / 2 ) = WK
                         AP( J+K*( 2*N-K-1 ) / 2 ) = WKP1
      100             CONTINUE
                   END IF
                END IF
             END IF
    *
    *        Store details of the interchanges in IPIV
    *
             IF( KSTEP.EQ.1 ) THEN
                IPIV( K ) = KP
             ELSE
                IPIV( K ) = -KP
                IPIV( K+1 ) = -KP
             END IF
    *
    *        Increase K and return to the start of the main loop
    *
             K = K + KSTEP
             KC = KNC + N - K + 2
             GO TO 60
    *
          END IF
    *
      110 CONTINUE
          RETURN
    *
    *     End of CSPTRF
    *
          END