The following is from the book book Design of Analog CMOS Integrated Circuit, Page 278.
calculation of input resistance.
The book said
$$V_{out} = (g_{m1} + g_{mb1}) V_x R_D$$
However, according to KCL.
$$- \frac{V_{out}} {R_D} = -V_x (g_{m1} + g_{mb1}) + \frac{V_{out}} {\frac{1} {s C_1} + \frac{1} {s C_2}}$$