LEVCHUK, V. On the unconditional bases of cores generated by differential equations of the second order . Reports of the National Academy of Sciences of Ukraine, Kyiv, Ukraine, n. 3, p. 3–7, 2024. DOI: 10.15407/dopovidi2017.03.003. Disponível em: https://nasu-periodicals.org.ua/index.php/dp/article/view/3323. Acesso em: 22 aug. 2025.