POPESCU, G. Positive definite kernels on free product semigroups and universal algebras. MATHEMATICA SCANDINAVICA, [S. l.], v. 84, n. 1, p. 137–160, 1999. DOI: 10.7146/math.scand.a-13938. Disponível em: https://www.mscand.dk/article/view/13938. Acesso em: 3 may. 2024.