High-order extremal principles and P-factor penalty function method for solving irregular optimization problems. - Ewa Szczepanik; Alexey Tret'yakov (Ann. UMCS Informatica, 2004)

p-th Order Optimality Conditions for Singular Lagrange Problem in Calculus of Variations. Elements of p-Regularity Theory. - Agnieszka Prusinska; Ewa Szczepanik; Alexey Tret'yakov (System Modelling and Optimization, 2011)