MATHEMATICS, cilt.7, 2019 (SCI-Expanded)
The purpose of the present work is to determine a bound for the functional H-2(2) = a(2)a(4) - a(3)(2) for functions belonging to the class C-Sigma of bi-close-to-convex functions. The main result presented here provides much improved estimation compared with the previous result by means of different proof methods than those used by others.