I am reading the article ENVELOPING ALGEBRAS OF PRELIE ALGEBRAS, SOLOMON IDEMPOTENTS AND THE MAGNUS FORMULA of Frédéric Chapoton and Frédéric Patras. Many definitions and results used in this article I do not know yet. In particular, I do not understand what means classical polarization argument in the following sentence on page 5:
" [...] Moreover, by the classical polarization argument, setting $a:=l_1+ \cdots l_n$ we get that $l_1\ldots l_n$ " is the $l_1, \ldots, l_n$-multilinear component of $\frac{a^n}{n!};$ [...] "
In which, $L$ is a pre-Lie algebra, $l_1, \ldots, l_n \in L$ and $l_1 \cdots l_n \in S(L)$, the symmetric algebra of $L$.
Can anyone suggest a reference that talk about this "polarization argument"?