To cite package ‘monotonicity’ in publications use:

  Köstlmeier S (2019). _monotonicity: Test for Monotonicity in Expected
  Asset Returns, Sorted by Portfolios_. R package version 1.3.1,
  <https://github.com/skoestlmeier/monotonicity>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {monotonicity: Test for Monotonicity in Expected Asset Returns, Sorted by
Portfolios},
    author = {Siegfried Köstlmeier},
    year = {2019},
    note = {R package version 1.3.1},
    url = {https://github.com/skoestlmeier/monotonicity},
  }