\begin{definition}[Definition:Primitive Prime Factor]
Let $\left\langle{a_n}\right\rangle$ be an integer sequence.
A '''primitive prime factor''' for a term $a_n$ is a prime number $p$ of $a_n$ such that:
:$p \divides a_n$
:$\nexists k \in \Z_{>0}: k < n: p \divides a_k$
where $a \divides b$ denotes that $a$ is a divisor of $b$.
That is, a prime factor of $a_n$ but of no preceding terms.
\end{definition}