SION array, dimension (N)
          The scalar factors of the elementary reflectors used.

</PRE>
<B>Workspace</B>
<PRE>
  DWORK   DOUBLE PRECISION array, dimension (N)

</PRE>
<A name="Method"><B><FONT SIZE="+1">Method</FONT></B></A>
<PRE>
  The routine uses N Householder transformations exploiting the zero
  pattern of the block matrix.  A Householder matrix has the form

                                  ( 1 ),
     H  = I - tau *u *u',    u  = ( v )
      i          i  i  i      i   (  i)

  where v  is