\,\mbox{R}(z, dt) = \exp\left[- \frac{i}{h}\ (x p_y - y p_x) dt\right]= \exp\left(- \frac{i}{h}\ l_z dt\right) = 1 - \frac{i}{h} l_z dt + .. .
To get a rotation for the angle \,t, we construct the following differential equation using the condition \mbox{R}(z, 0) = 1: .