Pinheiro J, Bates D, DebRoy S, Sarkar D and R Core Team (2016). nlme: Linear and Nonlinear Mixed Effects Models. R package version 3.1-125, http://CRAN.R-project.org/package=nlme.

Corresponding BibTeX entry:

  @Manual{,
    title = {{nlme}: Linear and Nonlinear Mixed Effects Models},
    author = {Jose Pinheiro and Douglas Bates and Saikat DebRoy and
      Deepayan Sarkar and {R Core Team}},
    year = {2016},
    note = {R package version 3.1-125},
    url = {http://CRAN.R-project.org/package=nlme},
  }