sed on personal preference. Make sure you use consistent notations, and state clearly what your notations mean.</span></div><div  class = 'S1'><span>In general, the letter </span><span style="font-family: STIXGeneral, STIXGeneral-webfont, serif; font-style: italic; font-weight: 400; color: rgb(0, 0, 0);">f</span><span> represents a particular rule of mapping, if there are multiple functional relationships in an economic model, different letters could be used, for example:</span></div><div  class = 'S1'><spa