Geodesic
On the superstability of the cosine and sine type functional equations
Annales Universitatis Paedagogicae Cracoviensis. Studia Mathematica, no. 15 (2016).

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In this paper, we study the superstablity problem of the cosine and sine type functional equations:f(xσ(y)a)+f(xya)=2f(x)f(y) f(xσ (y)a) + f(xya) = 2f(x)f(y)andf(xσ(y)a)−f(xya)=2f(x)f(y), f(xσ (y)a) - f(xya) = 2f(x)f(y),where f : S → ℂ is a complex valued function; S is a semigroup; σ is an involution of S and a is a fixed element in the center of S.
Keywords: functional equation, superstablity, stablity 
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Lehlou, Fouad; Moussa, Mohammed; Roukbi, Ahmed; Kabbaj, Samir. On the superstability of the cosine and sine type functional equations. Annales Universitatis Paedagogicae Cracoviensis. Studia Mathematica, no. 15 (2016). http://geodesic.mathdoc.fr/item/AUPCM_2016_15_a6/