by Akça, H., Covachev, V., Covacheva, Z.
Published in Journal of mathematical sciences (New York, N.Y.) (09.03.2015)

by Bainov, D.D., Covachev, V.C., Stamova, I.M.
Published in Journal of mathematical analysis and applications (01.11.1994)

by Bainov, D D, Covachev, V Ch
Published in Journal of physics. A, Mathematical and general (21.08.1994)

by Risteski, Ice, Covachev, Valéry
Year of Publication 2001

by BAINOV, D, COVACHEV, V, STAMOVA, I
Published in Annales de la faculté des sciences de Toulouse. Mathématiques (1991)