  There are a couple of purposes in this paper: to study a problem of
approximation with exponential functions and to show its relevance for the
economic science. We present results that completely solve the problem of the
best approximation by means of exponential functions and we will be able to
determine what kind of data is suitable to be fitted. Data will be approximated
using TAC (implemented in the R-package nlstac), a numerical algorithm for
fitting data by exponential patterns without initial guess designed by the
authors. We check one more time the robustness of this algorithm by
successfully applying it to two very distant areas of economy: demand curves
and nonlinear time series. This shows TAC's utility and highlights how far this
algorithm could be used.
