Hyers-Ulam stability of a generalized Apollonius type quadratic mapping

Let X,Y be linear spaces. It is shown that if a mapping Q:X→Y satisfies the following functional equation:(0.1)Q((∑i=1nzi)−(∑i=1nxi))+Q((∑i=1nzi)−(∑i=1nyi))=12Q((∑i=1nxi)−(∑i=1nyi))+2Q((∑i=1nzi)−(∑i=1nxi)+(∑i=1nyi)2) then the mapping Q:X→Y is quadratic. We moreover prove the Hyers-Ulam stability of the functional equation (0.1) in Banach spaces.