I am wanting to get the expression up to ##O(\epsilon^{2}) ## :
To show that ##\frac{1}{2w_{k}} (\frac{1}{w_{k}-k_{0}-i\epsilon} + \frac{1}{w_{k}+k_{0}-i\epsilon})##
##=##
## \frac{1}{k_{v}k^{v} + m^{2} - i\epsilon}##, [2]
where ##w_{k}^{2}=k^{2}+m^{2}##, ##k## the variable, and (this seemed...