program normal
*     this simple code tests the derivation for normal derivative of
*     log| x - y |
      double precision x1,x2,y1,y2,th,r,pi,h,dx,dy,r1,r2
      print *,'enter x1,x2,y1,y2,th,h'
      read *,x1,x2,y1,y2,th,h
      pi = 4.D0*datan(1.D0)
*
*     method 1:
      r = dsqrt((x1-y1)**2+(x2-y2)**2)
      de1 = ((x1-y1)*dsin(th)-(x2-y2)*dcos(th))/r/r
*
*     method 2:
      dx = h*dsin(th)
      dy = -h*dcos(th)
      xp = x1 + dx
      yp = x2 + dy
      xm = x1 - dx
      ym = x2 - dy
      r1 = dsqrt((xp-y1)**2+(yp-y2)**2)
      r2 = dsqrt((xm-y1)**2+(ym-y2)**2)
      de2 = 0.5D0*(dlog(r1)-dlog(r2))/h
*
      print *,'de1=',de1,' de2=',de2
      end