SPBEQU - compute row and column scalings intended to equi­
       librate a symmetric positive definite band  matrix  A  and
       reduce its condition number (with respect to the two-norm)


SYNOPSIS

       SUBROUTINE SPBEQU( UPLO, N, KD, AB, LDAB, S, SCOND,  AMAX,
                          INFO )

           CHARACTER      UPLO

           INTEGER        INFO, KD, LDAB, N

           REAL           AMAX, SCOND

           REAL           AB( LDAB, * ), S( * )


PURPOSE

       SPBEQU  computes row and column scalings intended to equi­
       librate a symmetric positive definite band  matrix  A  and
       reduce  its  condition  number  (with  respect to the two-
       norm).   S   contains   the   scale   factors,   S(i)    =
       1/sqrt(A(i,i)),  chosen  so  that the scaled matrix B with
       elements B(i,j) = S(i)*A(i,j)*S(j) has ones on the  diago­
       nal.   This  choice  of  S  puts the condition number of B
       within a factor N of the smallest possible condition  num­
       ber over all possible diagonal scalings.


ARGUMENTS

       UPLO    (input) CHARACTER*1
               = 'U':  Upper triangular of A is stored;
               = 'L':  Lower triangular of A is stored.

       N       (input) INTEGER
               The order of the matrix A.  N >= 0.

       KD      (input) INTEGER
               The  number  of  superdiagonals of the matrix A if
               UPLO = 'U', or the number of subdiagonals if  UPLO
               = 'L'.  KD >= 0.

       AB      (input) REAL array, dimension (LDAB,N)
               The  upper or lower triangle of the symmetric band
               matrix A, stored in the first  KD+1  rows  of  the
               array.  The j-th column of A is stored in the j-th
               column of the array AB as follows: if UPLO =  'U',
               AB(kd+1+i-j,j)  = A(i,j) for max(1,j-kd)<=i<=j; if
               UPLO  =   'L',   AB(1+i-j,j)      =   A(i,j)   for
               j<=i<=min(n,j+kd).

       LDAB     (input) INTEGER
                The  leading  dimension  of the array A.  LDAB >=
                KD+1.

               If INFO = 0, S contains the scale factors for A.

       SCOND   (output) REAL
               If INFO = 0, S contains the ratio of the  smallest
               S(i)  to  the  largest  S(i).  If SCOND >= 0.1 and
               AMAX is neither too large nor too small, it is not
               worth scaling by S.

       AMAX    (output) REAL
               Absolute value of largest matrix element.  If AMAX
               is very close to overflow or very close to  under­
               flow, the matrix should be scaled.

       INFO    (output) INTEGER
               = 0:  successful exit
               < 0:  if INFO = -i, the i-th argument had an ille­
               gal value.
               > 0:  if INFO = i, the i-th  diagonal  element  is
               nonpositive.


Man(1) output converted with man2html