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NAME
     chptrf - compute the factorization of  a  complex  Hermitian
     packed  matrix  A  using the Bunch-Kaufman diagonal pivoting
     method

SYNOPSIS
     SUBROUTINE CHPTRF( UPLO, N, AP, IPIV, INFO )

     CHARACTER UPLO

     INTEGER INFO, N

     INTEGER IPIV( * )

     COMPLEX AP( * )



     #include <sunperf.h>

     void chptrf(char uplo, int n, complex *cap, int *ipivot, int
               *info) ;

PURPOSE
     CHPTRF computes the factorization  of  a  complex  Hermitian
     packed  matrix  A  using the Bunch-Kaufman diagonal pivoting
     method:

        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.


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  Her-
               mitian  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  mul-
               tipliers  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  struc-
               ture  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  ille-
               gal 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  divi-
               sion  by  zero will occur if it is used to solve a
               system of equations.

FURTHER DETAILS
     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).

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