To cite cfda, use:

  Preda C, Grimonprez Q, Vandewalle V. Categorical Functional Data
  Analysis. The cfda R Package. Mathematics. 2021; 9(23):3074.
  https://doi.org/10.3390/math9233074

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Categorical Functional Data Analysis. The cfda R Package},
    author = {{Cristian Preda} and {Quentin Grimonprez} and {Vincent Vandewalle}},
    journal = {Mathematics},
    year = {2021},
    volume = {9},
    number = {23},
    url = {https://www.mdpi.com/2227-7390/9/23/3074},
    doi = {10.3390/math9233074},
  }