ZHETRF computes the factorization of a complex Hermitian matrix A


 using the Bunch-Kaufman diagonal pivoting method.  The form of the


 factorization is


    A = U*D*U**H  or  A = L*D*L**H


 where U (or L) is a product of permutation and unit upper (lower)


 triangular matrices, and D is Hermitian and block diagonal with


 1-by-1 and 2-by-2 diagonal blocks.


 This is the blocked version of the algorithm, calling Level 3 BLAS.

UPLO

          UPLO is CHARACTER*1


          = 'U':  Upper triangle of A is stored;


          = 'L':  Lower triangle of A is stored.

N

          N is INTEGER


          The order of the matrix A.  N >= 0.

A

          A is COMPLEX*16 array, dimension (LDA,N)


          On entry, the Hermitian matrix A.  If UPLO = 'U', the leading


          N-by-N upper triangular part of A contains the upper


          triangular part of the matrix A, and the strictly lower


          triangular part of A is not referenced.  If UPLO = 'L', the


          leading N-by-N lower triangular part of A contains the lower


          triangular part of the matrix A, and the strictly upper


          triangular part of A is not referenced.


          On exit, the block diagonal matrix D and the multipliers used


          to obtain the factor U or L (see below for further details).

LDA

          LDA is INTEGER


          The leading dimension of the array A.  LDA >= max(1,N).

IPIV

          IPIV is 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.

WORK

          WORK is COMPLEX*16 array, dimension (MAX(1,LWORK))


          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

LWORK

          LWORK is INTEGER


          The length of WORK.  LWORK >=1.  For best performance


          LWORK >= N*NB, where NB is the block size returned by ILAENV.

INFO

          INFO is 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.

Univ. of Tennessee

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  If UPLO = 'U', then A = U*D*U**H, 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**H, 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).