To cite package 'FuzzyNumbers' in publications use:

Gagolewski M., Caha J. (2019). FuzzyNumbers Package: Tools to Deal with Fuzzy Numbers in R. http://www.gagolewski.com/software/.

Corresponding BibTeX entry:

  @Manual{,
    title = {FuzzyNumbers Package: Tools to Deal with Fuzzy Numbers in
      R},
    author = {Marek Gagolewski and Jan Caha},
    year = {2019},
    url = {http://www.gagolewski.com/software/},
  }