subroutine poles(r1,c1,r2,c2)
      real r1,c1,r2,c2
      real tau1,tau2,tau3
      real pole(2)
      tau1=r1*c1
      tau2=r2*c2
      tau3=r1*c2
      a=-1
      b=(1/tau1+1/tau2+1/tau3)
      c=1/(tau1*tau2) 
      d=sqrt(b**2+4*a*c)
      pole(1)=(b+d)/(2*a)
      pole(2)=(b-d)/(2*a)
      print *, 'poles are ',pole
      print *, 'time constants are ',1/pole(1),1/pole(2)
      end