c *************************************************
c spectrum.f  
c
c      gcc-4.1.0 compliation command:
c     
c               gfortran spectrum.f
c
c *************************************************
      parameter(eta=0.01)
      parameter(x0=4.0)
      parameter(n0=100)
c declare some variables complex.
      complex g,z,zz
c open output file.
      open(7,file="output.dat",status="new")
c compute frequency increment.
      x=n0
      dx=x0/x
c compute number of bins.
      n=n0*2
c loop over frequencies.
      do 2 j=1,n
      x=dx*(j-n0-0.5)
c for this frequency compute green's function.
      z=cmplx(x,eta)
      zz=z**2-2.0
      g=csqrt(zz**2-4.0)
      y=aimag(g)*x
      if(y.gt.0.0) g=-g
      g=(g+zz)/2.0
c write result to file.
      write(7,1) x,g
    1 format(3f10.4)
    2 continue
      stop
      end