OICC Press Mathematical Analysis and its Contemporary Applications 1 1 2019 01 01 Non-stabilities of mixed type Euler-Lagrange k-cubic-quartic functional equation in various normed spaces 10.30495/maca.2019.679849 EN John Michael Rassias Pedagogical Department E.E., Section of Mathematics and Informatics, National and Capodistrian University of Athens, 4 Agamemnonos Street, Aghia Paraskevi, Athens 15342, Greece Mohan Arunkumar Department of Mathematics, Government Arts College, Tiruvannamalai-606603, Tamil Nadu, India Elumalai Sathya Department of Mathematics, Shanmuga Industries Arts and Science College, Tiruvannamalai-606603, Tamil Nadu, India Journal Article 2019 01 01 In this paper, we introduce and examine the generalized Ulam-Hyers stability of fixed Euler-Lagrange k-Cubic-Quartic functional Equation f(x + ky) + f(kx + y) + f(x - ky) + f(y - kx) = k^2 [2f(x + y) + f(x - y) + f(y - x)] + 2(k^4 - 1)[f(x) + f(y)] + k^2/4 (k^2 - 1)[f(2x) + f(2y)] where k is a real number with k ≠ 0, ±1 in various Banach spaces with the help of two different methods.