source: PSPA/parmelaPSPA/trunk/elispdp.f @ 412

      subroutine elipsdp(nt90,n,x,y,a,b,xc,yc,e)
c
c      subroutine for parmila to fit an elipse to a set of phase space
c      points.
c      n = no of points
c      x,y = coordinates
c      a,b = alpha & beta of ellipse
c      xc,yc = mean for x and y
c      e = emittance
c      e = gamma*x**2 + 2*alpha*x*y + beta*y**2
c      note that we do not let the centroid shift from the minimum e value
c
c--------------------------------------------------------------------------
      implicit double precision (a,b,e,f,r,s,x,y)
c
      include 'ucom.h'
c
      dimension x(n),y(n)
c--------------------------------------------------------------------------
c*
      fx=0.
      fy=0.
      sx=0.
      sy=0.
      sxy=0.
c      minimize e
      do 10 i = 1,n
      fx = fx + x(i)
      fy = fy + y(i)
      sx = sx + x(i)*x(i)
      sxy = sxy + x(i)*y(i)
      sy = sy + y(i)*y(i)
 10   continue
      xc = fx/n
      yc = fy/n
      scx = sx/n - xc*xc
      scy = sy/n - yc*yc
      scxy = sxy/n - xc*yc
      rbm=(scx*scy - scxy*scxy)
      if(rbm.le.0.) then
       if(nt90.eq.1) then
       write(nemit,*)
     *' no calculation of emittances for (x,xp): negative square root'
       else
       write(nemit,*)
     *' no calculation of emittances for (y,yp): negative square root'
       endif
      else
      e = dsqrt(rbm)
           a = -scxy/e
           b = scx/e
      endif
      return
      end
c++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*