To cite evtree in publications use:

  Grubinger T, Zeileis A, Pfeiffer K (2014). “evtree: Evolutionary
  Learning of Globally Optimal Classification and Regression Trees in
  R.” _Journal of Statistical Software_, *61*(1), 1-29.
  doi:10.18637/jss.v061.i01 <https://doi.org/10.18637/jss.v061.i01>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{evtree}: Evolutionary Learning of Globally Optimal Classification and Regression Trees in {R}},
    author = {Thomas Grubinger and Achim Zeileis and Karl-Peter Pfeiffer},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {61},
    number = {1},
    pages = {1--29},
    doi = {10.18637/jss.v061.i01},
  }