subroutine  drotf (n,dx,incx,dy,incy,c,s)

c

c     applies a plane rotation.

c     jack dongarra, linpack, 3/11/78.

c     modified 12/3/93, array(1) declarations changed to array(*)

c

      double precision dx(*),dy(*),dtemp,c,s

      integer i,incx,incy,ix,iy,n

c

      if(n.le.0)return

      if(incx.eq.1.and.incy.eq.1)go to 20

c

c       code for unequal increments or equal increments not equal

c         to 1

c

      ix = 1

      iy = 1

      if(incx.lt.0)ix = (-n+1)*incx + 1

      if(incy.lt.0)iy = (-n+1)*incy + 1

      do 10 i = 1,n

        dtemp = c*dx(ix) + s*dy(iy)

        dy(iy) = c*dy(iy) - s*dx(ix)

        dx(ix) = dtemp

        ix = ix + incx

        iy = iy + incy

   10 continue

      return

c

c       code for both increments equal to 1

c

   20 do 30 i = 1,n

        dtemp = c*dx(i) + s*dy(i)

        dy(i) = c*dy(i) - s*dx(i)

        dx(i) = dtemp

   30 continue

      return

      end