Formal notation for representing decimal numbers What is the formal mathematical notation for representing a decimal number using variables as it's digits?     


I am honestly surprised that this has not been asked yet on MSE. Let me clarify a little using an example. Consider the $3$ digit decimal number $123$. Say I  have $3$ variables for each of the decimal places $a,b,c$ such that $$a=1$$ $$b=2$$ $$c=3$$
I know that I can represent the number using $$a\times 100+b\times 10 + c$$
But this is too tedious. All I want to know is if there is any shorthand notation developed specifically for this purpose.
I have faint memory of using some notation for this exactly ONCE in my life but I think my memory is playing tricks on me. Does anybody actually remember some notation?
Thanks



Note

Not to be misunderstood as asking for notations of summations, et cetera. I am asking for specific purpose notation only.
I have seen $\overline {abc}$ used by some people on this site.  Others will just write $abc$ and state that it means concatenating the digits instead of multiplication.  Usually it is clear what problems want it that way.
Alternatively, I have found another less convenient but formally correct symbol
$$100a+10b+c=a\|b\|c$$
This of course is also usable on numbers $$123=1\|2\|3$$
The "$\|$" (represented by \| in $L^AT_EX$) operation is called concatenation (as mentioned by @GitGud and @RossMillikan).
More can be read about this operation here