KRIŠČIŪNAITĖ, A. On contact umbolic hypersurface in O-pair space. Lithuanian Mathematical Journal, [S. l.], v. 11, n. 1, p. 117–124, 1971. DOI: 10.15388/LMJ.1971.20965. Disponível em: https://www.journals.vu.lt/LMJ/article/view/20965. Acesso em: 6 dec. 2025.