Let $(\mathcal A,\varphi)$ be a noncommutative probability space, and let $\kappa_n:\mathcal A^n\to\mathbb C$, for $n\geq 1$, be the free cumulants associated to $\varphi$ through the [Speicher moment-cumulant formula](/theorems/7128) over noncrossing partitions. For every $a_1,a_2,a_3,a_4\in\mathcal A$, the first four identities are
\begin{align*}
\varphi(a_1)=\kappa_1(a_1).
\end{align*}
\begin{align*}
\varphi(a_1a_2)=\kappa_2(a_1,a_2)+\kappa_1(a_1)\kappa_1(a_2).
\end{align*}
\begin{align*}
\varphi(a_1a_2a_3)=\kappa_3(a_1,a_2,a_3)+\kappa_2(a_1,a_2)\kappa_1(a_3)+\kappa_2(a_1,a_3)\kappa_1(a_2)+\kappa_2(a_2,a_3)\kappa_1(a_1)+\kappa_1(a_1)\kappa_1(a_2)\kappa_1(a_3).
\end{align*}
\begin{align*}
\varphi(a_1a_2a_3a_4)=\kappa_4(a_1,a_2,a_3,a_4)+\kappa_3(a_1,a_2,a_3)\kappa_1(a_4)+\kappa_3(a_1,a_2,a_4)\kappa_1(a_3)+\kappa_3(a_1,a_3,a_4)\kappa_1(a_2)+\kappa_3(a_2,a_3,a_4)\kappa_1(a_1)+\kappa_2(a_1,a_2)\kappa_2(a_3,a_4)+\kappa_2(a_1,a_4)\kappa_2(a_2,a_3)+\kappa_2(a_1,a_2)\kappa_1(a_3)\kappa_1(a_4)+\kappa_2(a_1,a_3)\kappa_1(a_2)\kappa_1(a_4)+\kappa_2(a_1,a_4)\kappa_1(a_2)\kappa_1(a_3)+\kappa_2(a_2,a_3)\kappa_1(a_1)\kappa_1(a_4)+\kappa_2(a_2,a_4)\kappa_1(a_1)\kappa_1(a_3)+\kappa_2(a_3,a_4)\kappa_1(a_1)\kappa_1(a_2)+\kappa_1(a_1)\kappa_1(a_2)\kappa_1(a_3)\kappa_1(a_4).
\end{align*}