To cite cts in publications use:

  Zhu Wang (2013). cts: An R Package for Continuous Time Autoregressive
  Models via Kalman Filter. Journal of Statistical Software, 53(5),
  1-19. URL https://www.jstatsoft.org/v53/i05/.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{cts}: An {R} Package for Continuous Time Autoregressive Models via Kalman Filter},
    author = {{Zhu Wang}},
    journal = {Journal of Statistical Software},
    year = {2013},
    volume = {53},
    number = {5},
    pages = {1--19},
    url = {https://www.jstatsoft.org/v53/i05/},
  }