Direct sum of lower semi-frames in Hilbert spaces. Gulf Journal of Mathematics, [S. l.], v. 19, n. 2, p. 412–423, 2025. DOI: 10.56947/gjom.v19i2.2803. Disponível em: https://gjom.org/index.php/gjom/article/view/2803. Acesso em: 26 aug. 2026.