To cite package greekLetters in publications use:

Kévin Allan Sales Rodrigues (2020). greekLetters: routines for writing Greek letters and mathematical symbols on the RStudio and RGui. R package version 0.0.7.

Corresponding BibTeX entry:

  @Manual{,
    title = {greekLetters: routines for writing Greek letters and
      mathematical symbols on the RStudio and RGui},
    author = {{Kévin Allan Sales Rodrigues}},
    year = {2020},
    note = {R package version 0.0.7},
  }