def CutOnCondition(Kz):
    """Determine if mode is cuton based on wave number
    Must be real to propagate (pos. imag.: decay exponential, neg.: expand)
    Must be negative for upstream propagation
    Kz --> axial wavenumber for current mode
    """
    cuton = 'No'
    #Test if no imaginary part (propagation)
    if Kz.imag == 0:
        #Test if upstream propagation
        if Kz.real < 0:
            cuton = 'Yes'
    return cuton