Laxman Kumar Soni, Priyanka Soni, Daljeet Kaur, C. R. Choudhary, and K.C. Agarwal. “Correlation of Vitamin D Deficiency With Frequent Exacerbations in COPD Patients at Tertiary Care Hospital in Western Rajasthan”. International Journal of Health and Clinical Research, vol. 5, no. 1, Jan. 2022, pp. 586-8, https://www.ijhcr.com/index.php/ijhcr/article/view/4709.