KOENKER, R.; NG, P. SparseM: A Sparse Matrix Package for R *. Journal of Statistical Software, [S. l.], v. 8, n. 6, p. 1–9, 2003. DOI: 10.18637/jss.v008.i06. Disponível em: https://www.jstatsoft.org/index.php/jss/article/view/v008i06. Acesso em: 4 dec. 2021.