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?)

yes as it can be expressed as f(x)= 450975+1000*x when x $\in$ {0,1,..9}
Over the set of natural number, we still can have the formula 
f(x)= $45*1000*10^{(⌊log10x⌋+1)}+975+1000*x$ 
Proof: How many digits does a number have? $\lfloor \log_{10} n \rfloor +1$
