To cite package 'frontier' in publications use:

  Tim Coelli and Arne Henningsen (2020). frontier: Stochastic Frontier
  Analysis. R package version 1.1-9.
  https://CRAN.R-Project.org/package=frontier.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {frontier: Stochastic Frontier Analysis},
    author = {Tim Coelli and Arne Henningsen},
    year = {2020},
    note = {R package version 1.1-9},
    url = {https://CRAN.R-Project.org/package=frontier},
  }