I came across this problem when trying to solve the following integral equations arising in direct scattering: $$ \begin{align} n_{11}(x,z)=1+\int_{-\infty}^xe^{-izy}u(y)n_{21}(y,z)dy, \quad n_{21}(x,z)=\int_{-\infty}^xe^{izy}\bar{u}(y)n_{11}(y,z)dy \end{align} $$

I was suggested to iterate thoses two equations to obtain Volterra series representation. However I am not familiar with Volterra series, so is there anyone who can kindly provide me with some kind of recipes on how to do it? Thank you very much!