To cite spatstat in publications use:

Baddeley A, Rubak E, Turner R (2015). Spatial Point Patterns: Methodology and Applications with R. Chapman and Hall/CRC Press, London. https://www.routledge.com/Spatial-Point-Patterns-Methodology-and-Applications-with-R/Baddeley-Rubak-Turner/9781482210200/.

If you use hybrid models, please also cite:

Baddeley A, Turner R, Mateu J, Bevan A (2013). “Hybrids of Gibbs Point Process Models and Their Implementation.” Journal of Statistical Software, 55(11), 1–43. doi:10.18637/jss.v055.i11.

In survey articles, please cite the original paper on spatstat:

Baddeley A, Turner R (2005). “spatstat: An R Package for Analyzing Spatial Point Patterns.” Journal of Statistical Software, 12(6), 1–42. doi:10.18637/jss.v012.i06.

Corresponding BibTeX entries:

  @Book{,
    title = {Spatial Point Patterns: Methodology and Applications with
      {R}},
    author = {Adrian Baddeley and Ege Rubak and Rolf Turner},
    year = {2015},
    publisher = {Chapman and Hall/CRC Press},
    address = {London},
    url =
      {https://www.routledge.com/Spatial-Point-Patterns-Methodology-and-Applications-with-R/Baddeley-Rubak-Turner/9781482210200/},
  }

  @Article{,
    title = {Hybrids of Gibbs Point Process Models and Their
      Implementation},
    author = {Adrian Baddeley and Rolf Turner and Jorge Mateu and
      Andrew Bevan},
    journal = {Journal of Statistical Software},
    year = {2013},
    volume = {55},
    number = {11},
    pages = {1--43},
    doi = {10.18637/jss.v055.i11},
  }

  @Article{,
    title = {{spatstat}: An {R} Package for Analyzing Spatial Point
      Patterns},
    author = {Adrian Baddeley and Rolf Turner},
    journal = {Journal of Statistical Software},
    year = {2005},
    volume = {12},
    number = {6},
    pages = {1--42},
    doi = {10.18637/jss.v012.i06},
  }