Simple proofs of certain results on generalized Fekete-Szegő functional in the class \(\mathcal {S}\)

In this paper we give simple proofs for the main results concerning generalized Fekete-Szegő functional of type \(\left| a_{3}(f)-\lambda a_{2}(f)^{2}\right| -\mu |a_{2}(f)|\), where \(\lambda \in \mathbb {C}\), \(\mu >0\) and \(a_{n}(f)\) is n-th coefficient of the power series expansion of \(f\in \mathcal {S}\). In addition, we studied this functional separately for the class \(\mathcal {K}\) of convex functions and we emphasize that all the results of the paper are sharp (i.e. the best possible). The advantages of the present study are that the techniques used in the proofs are more easier and use known results regarding the univalent functions, and those that it give the best possible results not only for the entire class of univalent normalized functions \(\mathcal {S}\) but also for its subclass of convex functions \(\mathcal {K}\).