LOPOTKO, O. V. On the extension of even-positive-definite functions of one and two variables. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 51, n. 11, p. 1577–1581, 1999. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4761. Acesso em: 5 dec. 2025.