Alexander V. Zolotaryuk and Yaroslav Zolotaryuk, Point interactions with bound states: A zero-thickness limit of a double-layer heterostructure, Low Temp. Phys. 46, (2020) [Fiz. Nizk. Temp. 46, 927-933, (2020)] DOI: https://doi.org/10.1063/10.0001540.