Independence, Trade-Offs, and Transformations in Bivariate Utility Functions

This paper explores several important notions relevant to modern utility theory. Restricting the discussion to the consideration of bivariate utility functions, the paper defines and examines the interrelationships between (1) independence in the utility sense, (2) trade-off or indifference curves, and (3) transformation curves (as defined herein). Following the form in which a set of basic axioms of utility are stated, independence is defined in terms of indifference between 50-50 gambles, and it is shown that, if the condition of the definition holds, then φ(x, y), the bivariate utility function, can be written as a function of x plus a function of y. The concepts of trade-off curve (indifference curve) and transformation curve are also defined on the basis of the indifference relation but are not concerned with gambles. After exploring relationships among the three notions it is shown how the utility curves for the two variables under independence can be constructed on the basis of two trade-off curves.