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In ${\rm\LaTeX}$ typesetting, when we repeat a long and complex formula in long documents, it is appropriate to create a new command that just by calling this new command we get the desired output. For example, I have used the following math expression in my previous document frequently: $$\{a^1,a^2,\ldots,a^n\}$$ For doing this in usual way, we need to press 22 keys on keyboard (and think about $(\frac{\partial}{\partial x^1}, \cdots,\frac{\partial}{\partial x^n})$ and other terrible formulas). Of course we can do this by copy and paste from similar one in the text. it is much better to define the following new command on preamble



ewcommand{\set}[1]{\setaux#1\relax} \def\setaux#1#2#3\relax{% \{ {#1}#2 1, \ifnum\pdfstrcmp{#3}{3}=0 {#1}#2 2 \else \ldots \fi , {#1}#2{#3} \} }

and just by typing \set{a^n} in our text we get the same output.