This comes from Sedra's Microelectronic Circuits, example 17.1.
If I have a transfer function with a pole \$p_1 = \omega_0(-0.1736 + j0.9848)\$.
The example talks about "combining \$p_1\$ with its complex conjugate", becoming \$s^2+0.3472s + \omega_0^2\$.
By trial and error I found that this "combining with its complex-conjugate" assumes an expression \$s^2 + (p_1 + \overline{p_1})s + p_1 \overline{p_1}\$. But I don't get what this combination means exactly and why it needs to happen here.