A function of one variable $y = f(x)$. However, I suspect that the function does not need to necessarily be strictly mathematical.
If $x = 1$, then $y = 0$.
If $x = 2$, then $y = 0$.
If $x = 10$, then $y = 9$.
If $x = 3$, then $y = 3$.
If $x = 20$, then $y = 18$.
If $x = 1990$, then $y = 1989$.
What function is it? (A puzzle from an old informatics textbook which I was unable to solve for years.)
It looks like
$y = 3 \lfloor \frac{x}{3} \rfloor$
where
$\lfloor . \rfloor$ is the floor function indicating the greatest integer less than or equal to the argument.
Similarly
$y$ is the greatest integer $\leq x$ which is divisible by $3$.
Given that it comes from an informatics textbook Bubbler make an important point that
some programming is relevant here (and it must be C if it's from 1990):
int f(int x) { return x / 3 * 3; }
int f(int x) { return x / 3 * 3; } $\endgroup$ An alternative (and very similar) answer (which still fits):
$f(x) = x - L_{10}(x) \bmod 3$, where $L_{10}(x)$ is the leftmost (nonzero as usual) digit of $x$ written in decimal (i.e. $L_{10}(534)=5, L_{10}(6207)=6$ etc.). (The hexomino's answer is simply $f(x)=x-x\bmod3$).
