Questa nota riguarda la costruzione di operatori differenziali lineari
\[
T=\overset{n}{\underset{i=0}{\sum}}\overset{m}{\underset{k=0}{\sum}}a_{ik}(z,\zeta)\frac{\partial^{i+k}}{\partial z^{i}\partial\zeta^{k}},
\]
$(z,\zeta)\epsilon\mathbf{D\subset C^{\textrm{2}}}$che trasformano
tutte le soluzioni u (z, $\zeta$) di equazioni u$_{z\zeta}$+ a($z,\zeta$)u$\zeta$+
b$(z,\zeta)$u$_{z}$= 0 in soluzioni $\tilde{u}$=Tu di equazioni
$\tilde{u}_{z\zeta}$+$\tilde{a}$$(z,\zeta)$$\tilde{u_{\zeta}}$+$\tilde{b}$$(z,\zeta)\tilde{u_{\zeta}}$=0 \[
T=\overset{n}{\underset{i=0}{\sum}}\overset{m}{\underset{k=0}{\sum}}a_{ik}(z,\zeta)\frac{\partial^{i+k}}{\partial z^{i}\partial\zeta^{k}},
\]
$(z,\zeta)\epsilon\mathbf{D\subset C^{\textrm{2}}}$which map all
solutions u (z, $\zeta$) of equations u$_{z\zeta}$+ a($z,\zeta$)u$\zeta$+
b$(z,\zeta)$u$_{z}$= 0 into solutions $\tilde{u}$=Tu of equations
$\tilde{u}_{z\zeta}$+$\tilde{a}$$(z,\zeta)$$\tilde{u_{\zeta}}$+$\tilde{b}$$(z,\zeta)\tilde{u_{\zeta}}$=0
