  The first-order correlation function, which is accessible experimentally,
contains all essential information about the state of the system of
non-interacting electrons. Here I discuss how this function can be used to
answer the question whether the state of a periodic stream of single-electron
wave-packets is a multi-particle state or it is the product of single-particle
states. In the latter case the correlation function is expected to be
factorizable while in the former case it is not. As an example I consider a
train of Lorentzian in shape single-electron excitations, levitons. I
demonstrate that the correlation function in time domain is factorizable or not
depending on whether the wave-packets are separated or overlapping. In
contrast, the correlation function in energy domain is always factorizable and
thus cannot be used to distinguish single- and multi-particle states.
