Pham Huu Anh Ngoc Nguyen Khoa Son Stability Radii of Positive Linear Functional Differential Equations under Multi-Perturbations. 2278-2295 2005 43 SIAM J. Control. Optim. 6 https://doi.org/10.1137/S0363012903434789 db/journals/siamco/siamco43.html#NgocS05