Rasulov, Tulkin. β€œON THE ESSENTIAL SPECTRUM OF A QUADRATIC OPERATOR MATRIX OF ORDER 4”. Journal of Global Research in Mathematical Archives(JGRMA) 6, no. 10 (October 16, 2019): 17–20. Accessed March 14, 2025. https://jgrma.com/index.php/jgrma/article/view/551.