Let $R\subseteq S$ be an integral extension of commutative rings with identity. Let $P$ be a prime ideal in $R$ and $Q$ a prime ideal in $S$. If $Q=PS$ and $P=Q\cap R$ what can we say about $Q^n\cap R$? This ideal always contains $P^n$, but when does equality hold?
For example if $R\subseteq S$ is flat: the extension then actually is faithfully flat and thus for every ideal $I\subseteq R$ we have $IS\cap R=I$. Applying this to $I=P^n$ yields $$P^n = SP^n\cap R = (SP)^n \cap R = Q^n \cap R.$$
However flatness is not a frequent property of integral extensions ...