alculated the noise factor of 2-most amp

wwm101

in Federio Bruccoleri doctor degree’s paper
he assumes the MOST only have drain noise （channel noise）
then definitely　the drain noise I2n/△f=4KT*r*gm=4KT*NEF*gm

in follow schematic， he calculated the noise factor equal to 1+NEFa+4NEF(AVF-1)/AVF2 　　at ΔNEF=NEFb-NEFa　And gb+1/ZL>>0（gb+1/ZL=gb）　。

how to calculate it by hand?
Thank you in advance！