Before revision: The resulting so-called singular arc will be optimal if it satisfies the Kelley condition: .Bryson, Ho: Applied Optimal Control, Page 246 This condition is also called the generalized Legendre-Clebsch condition) .
After revision: The resulting so-called singular arc will be optimal if it satisfies the Kelley condition: Others refer to this condition as the generalized Legendre–Clebsch condition .