To cite pbkrtest in publications use:

Ulrich Halekoh, Søren Højsgaard (2014). A Kenward-Roger Approximation and Parametric Bootstrap Methods for Tests in Linear Mixed Models - The R Package pbkrtest. Journal of Statistical Software, 59(9), 1-30. URL http://www.jstatsoft.org/v59/i09/.

Corresponding BibTeX entry:

  @Article{,
    title = {A Kenward-Roger Approximation and Parametric Bootstrap
      Methods for Tests in Linear Mixed Models -- The {R} Package
      {pbkrtest}},
    author = {Ulrich Halekoh and S{\o}ren H{\o}jsgaard},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {59},
    number = {9},
    pages = {1--30},
    url = {http://www.jstatsoft.org/v59/i09/},
  }