Introduction to a theory of the internal structure of functional relationshipsW Leontief - Econometrica, Journal of the Econometric Society, 1947
http://www.jstor.org/stable/1905335




The internal structure of functional relationships: separability, substitution, and aggregationER Berndt, LR Christensen - The Review of Economic Studies, 1973
http://www.jstor.org/stable/2296459