An Eigenvalue Perturbation Approach to Stability Analysis, Part II: When Will Zeros of Time-Delay Systems Cross Imaginary Axis?
Jie Chen, Peilin Fu, Silviu-Iulian Niculescu, and Zhihong Guan
SIAM J. on Control and Optimization / Volume 48 / Issue 8
http://epubs.siam.org/sicon/resource/1/sjcodc/v48/i8/p5583_s1?isAuthorized=no