Let
C be the familiar class of normalized close-to-convex functions in the unit disk. In [17], Koepf demonstrated that, as to a function
f(ξ)=ξ+∞∑m=2amξm in the class
C,
maxf∈C|a3−λa22|≤{3−4λ,λ∈[0,13],13+49λ,λ∈[13,23],1,λ∈[23,1].
By applying this inequality, it can be proven that
||a3|−|a2||≤1 for close-to-convex functions. Now we generalized the above conclusions to a subclass of close-to-starlike mappings defined on the unit ball of a complex Banach space.