618 PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA -

But, most interesting of all, this is the case to which macroeconomic
 models of growth can be correctly applied. Since the
system expands keeping its proportions constant, there is in
fact no loss of generality in framing the analysis in macroeconomic
 terms. The usual convention that the variables must
be considered as measured in terms of a composite commodity,
made up of a fixed « basket of goods », is in this case perfectly
legitimate and logically unobjectionable. As all our results
have shown, since all single coefficients remain constant through
time, the composition or structure of the system, once specified
at time zero, remains the same for all time.

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