Badea, C., Badea, I., & Cottin, C. (1988). A Korovkin-type theorem for generalizations of boolean sum operators and approximation by trigonometric pseudopolynomials.
Anal. Numér. Théor. Approx.,
17(1), 7-17.
https://ictp.acad.ro/jnaat/journal/article/view/1988-vol17-no1-art2