Hyers-Ulam stability of a generalized trigonometric-quadratic functional equation

The Hyers-Ulam stability of the generalized trigonometric-quadratic functional equation ( ) ( ) ( ) ( ) ( ) ( ) 2 F x y G x y H x K y L x M y + − − = + + over the domain of an abelian group and the range of the complex field is established based on the assumption of the unboundedness of the function K. Subject to certain natural conditions, explicit shapes of the functions H and K are determined. Applications to several existing related results are direct consequences. Key-Words: Trigonometric functional equations, quadratic functional equations, abelian group, unboundedness, additive function, Hyers-Ulam stability.