Solution
= Solution
For <functions> $f,g:\mathbb N\to\mathbb R$, <big O notation> means
$$
\boxed{f=O(g)\iff\exists C>0\ \exists N\ \forall n\ge N:
|f(n)|\le C|g(n)|.}
$$
Using the <natural logarithm> and <exponential function>, $3^{\log n}=n^{\log3}$. Since $\log3>1$, for every $n\ge1$ we have $n\le n^{\log3}$. The witnesses $C=1,N=1$ therefore prove
$$
\boxed{n=O(3^{\log n}).}
$$