Please cite R.oo/R.methodsS3 as

Bengtsson, H. The R.oo package - Object-Oriented Programming with References Using Standard R Code, Proceedings of the 3rd International Workshop on Distributed Statistical Computing (DSC 2003), ISSN 1609-395X, Hornik, K.; Leisch, F. & Zeileis, A. (ed.), 2003

Corresponding BibTeX entry:

@InProceedings{, author = {Henrik Bengtsson}, title = {The {R.oo} package - Object-Oriented Programming with References Using Standard {R} Code}, booktitle = {Proceedings of the 3rd International Workshop on Distributed Statistical Computing (DSC 2003)}, year = {2003}, editor = {Kurt Hornik and Friedrich Leisch and Achim Zeileis}, address = {Vienna, Austria}, month = {March}, issn = {1609-395X}, url = {https://www.r-project.org/conferences/DSC-2003/Proceedings/Bengtsson.pdf}, howpublished = {https://www.r-project.org/conferences/DSC-2003/Proceedings/}, }