In a function formula, can the independant variable $x$ ( ranging over $N$) be used as a simple figure inside a number? E.g. $ f(x)= 4,5x975$ Suppose $x$ ranges over {0,1,2,3,4,5,6,7,8,9}. 
Is $f(x) = 4,5x975$ a valid function formula ? 
What about the case where $x$ ranges over the set of natural numbers. 
In that case, $x$ would not represent necessarily the hundredth, 9 would not either necessarily represent the $10^{-3} $th, etc. 
Which equivalent general formula would give us the value of $f(x)$ in case $x\gt9$? ( I mean, the case where $x$ has 2 digits or more?)

This is really just a notation question. If you're clear about what you mean with this notation ahead of time, then it's probably safe to use it as long as you're not using the standard notation side-by-side with this. This could cause confusion when the two notations collide, e.g. $10x$ is either a three-digit number with unknown ones digit or the product of $10$ and an unknown number.
