Kaiguang, Z., BA, M.- ting, MENG, H.- ling, & SUN, Y.- min. (2019). Spatial Distribution Characteristics and Evolution Pattern of Air Quality in Henan Province. Journal of Progressive Research in Mathematics, 15(1), 2564-2568. Retrieved from http://scitecresearch.com/journals/index.php/jprm/article/view/1712