To cite spca in publications use:

  Merola G (2015). “Least Squares Sparse Principal Component Analysis:
  a Backward Elimination approach to attain large loadings.” _Australia
  & New Zealand Journal of Statistics_, *57*, 391-429.
  doi:10.1111/anzs.12128 <https://doi.org/10.1111/anzs.12128>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Least Squares Sparse Principal Component Analysis: a Backward Elimination approach to attain large loadings},
    author = {Giovanni Maria Merola},
    journal = {Australia & New Zealand Journal of Statistics},
    year = {2015},
    volume = {57},
    pages = {391--429},
    doi = {10.1111/anzs.12128},
  }

  Merola G, Chen G (2019). “Projection sparse principal component
  analysis: An efficient least squares method.” _Journal of
  Multivariate Analysis_, *173*, 366-382.
  doi:10.1016/j.jmva.2019.04.001
  <https://doi.org/10.1016/j.jmva.2019.04.001>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Projection sparse principal component analysis: An efficient least squares method},
    author = {Giovanni Maria Merola and Gemai Chen},
    journal = {Journal of Multivariate Analysis},
    year = {2019},
    volume = {173},
    pages = {366--382},
    doi = {10.1016/j.jmva.2019.04.001},
  }