In the book by Hardy Orders of Infinity you will find the Theorem (p. 18)
Any L-function is ultimately continuous, of constant sign, and monotonic, and, as $x\to\infty$, tends to $\infty$, or to zero or to some other definite limit. Further, if $f$ and $\phi$ are L-functions, one or other of the relations $f\succ\phi$, $f\asymp\phi$, $f\prec\phi$ holds between them.
With the definition:
We define a \emph{logarithmic-exponential function} (shortly, an L-function) as a real one valued
function defined, for all values of $x$ greater than some definite value, by a finite combination
of the ordinary algebraical symbols (viz. $+$, $-$, $\times$, $\div$, $\root n \of \cdot$ ) and the functional symbols $\log(\cdots)$ and $e^{(\dots)}$, operating on the variable $x$ and on real constants.