On a generalization of a class of production functions

In Appl. Econ. Lett. 18 (2011), 1777–1784, as a natural generalization of some famous production models with two inputs, C.A. Ioan and G. Ioan introduced a new class of production functions with constant return to scale, called sum production function, and proved three theorems of characterization for such production models. In this article, we give new and more simple proofs of these theorems, extending also the results to the case of increased/decreased return to scale. The generalization to the case of an arbitrary number of inputs is also discussed.