'Advanced topics' (Electromagnetic Waves, 2017), 'Adjoint Sensitivity Analysis of High Frequency Structures with MATLAB®', Chap. 10, pp. 247-261, DOI: 10.1049/SBEW525E_ch10
IET Digital Library, https://digital-library.theiet.org/;jsessionid=16kq3euh5ou3j.x-iet-live-01content/books/10.1049/sbew525e_ch10