and we write y=f(x)
then we say we have written the function on the left.
Another notation is to write y=xf and say we have written the
function on the right.
You have probably seen both but perhaps have not realized it. Before we give
some examples, let's also introduce some more notation. If y in the
codomain is such that y=f(x) for x in the domain, then we write
.
Consider the function 
defined by 
.
This function (as we have shown) is typically written on the left.
Following the same principle, then what would happen with the squaring function?
We'd have 
defined by 
by writing
the function on the left. If, though, we write the function on the right we
would get the more familiar 
.
We don't wish to imply writing functions on the left is preferable to writing functions on the right or vice-versa. There are advantages and disadvantages to both notations.
Some other functions that are written on the right include the factorial
function ( 
) and applications of functions on a calculator (for
example you probably enter then number first and then press the function key).