To cite etm in publications use:

  Arthur Allignol, Martin Schumacher, Jan Beyersmann (2011). Empirical
  Transition Matrix of Multi-State Models: The etm Package. Journal of
  Statistical Software, 38(4), 1-15. URL
  https://www.jstatsoft.org/v38/i04/.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Empirical Transition Matrix of Multi-State Models: The {etm} Package},
    author = {Arthur Allignol and Martin Schumacher and Jan Beyersmann},
    journal = {Journal of Statistical Software},
    year = {2011},
    volume = {38},
    number = {4},
    pages = {1--15},
    url = {https://www.jstatsoft.org/v38/i04/},
  }