Martynyuk, A. A., and Chzhen-tsi Sun'. 1999. “On the Practical $μ$-Stability of Solutions of Standard Systems With Delay”.
Ukrains’kyi Matematychnyi Zhurnal
51 (2): 204–213.
https://umj.imath.kiev.ua/index.php/umj/article/view/4599
.