'The adjoint variable method for frequency-independent constitutive parameters' (Electromagnetic Waves, 2017), 'Adjoint Sensitivity Analysis of High Frequency Structures with MATLAB®', Chap. 3, pp. 49-98, DOI: 10.1049/SBEW525E_ch3
IET Digital Library, https://digital-library.theiet.org/;jsessionid=51g0lk9on4d7p.x-iet-live-01content/books/10.1049/sbew525e_ch3