To cite NPCirc in publications use:

Oliveira M, Crujeiras RM, Rodríguez-Casal A (2014). “NPCirc: An R Package for Nonparametric Circular Methods.” Journal of Statistical Software, 61(9), 1–26. https://www.jstatsoft.org/v61/i09/.

Corresponding BibTeX entry:

  @Article{,
    title = {{NPCirc}: An {R} Package for Nonparametric Circular
      Methods},
    author = {Mar{\'i}a Oliveira and Rosa M. Crujeiras and Alberto
      Rodr{\'i}guez-Casal},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {61},
    number = {9},
    pages = {1--26},
    url = {https://www.jstatsoft.org/v61/i09/},
  }