Kangro, Ilmārs, et al. “SPECIAL HPERBOLIC TYPE APPROXIMATION FOR SOLVING OF 3-D TWO LAYER STATIONARY DIFFUSION PROBLEM”. ENVIRONMENT. TECHNOLOGY. RESOURCES. Proceedings of the International Scientific and Practical Conference, vol. 3, June 2019, pp. 95-100, https://doi.org/10.17770/etr2019vol3.4079.