Given a function $a(x)$ is there a general method to writing a function $f(x)$ so that is has the asymptote $a(x)$?
As far as I can tell from here you can sort of do this as long as $a(x)$ is a polynomial and you don't mind vertical asymptotes. Then $f(x)=a(x) + 1/x$ does the trick. But what about doing it without vertical asymptotes and for more general $a(x)$?