@article{ullah_shah_2013,  title={Half-factoriality in subrings of trigonometric polynomial rings},  volume={14},  ISSN={2179-8451},  url={https://doi.org/10.1590/S2179-84512013005000003},  DOI={10.1590/S2179-84512013005000003},  number={2},  journal={TEMA (São Carlos)},  publisher={Sociedade Brasileira de Matemática Aplicada e Computacional},  author={Ullah, Ehsan and Shah, Tariq},  year={2013},  month={May},  pages={173–186} }