Shavarovsky , B. Z. “On Decomposability of Matrix Polynomials With Coefficients of Quasi-Simple Structure”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 44, no. 12, Dec. 1992, pp. 1716-8,
https://umj.imath.kiev.ua/index.php/umj/article/view/8197
.