If you want to cite the cptcity R package, please use:
Ibarra-Espinosa S (2017). cptcity: incorporating the cpt-city archive into R. Colour gradients from http://soliton.vm.bytemark.co.uk/pub/cpt-city/, https://CRAN.R-project.org/package=vein.
Corresponding BibTeX entry:
@Manual{, title = {cptcity: incorporating the cpt-city archive into R}, author = {Sergio Ibarra-Espinosa}, year = {2017}, note = {Colour gradients from http://soliton.vm.bytemark.co.uk/pub/cpt-city/}, url = {https://CRAN.R-project.org/package=vein}, }