Blaga, P., and Gh. Coman. “On Some Bivariate Spline Operators”.
Anal. Numér. Théor. Approx.
, vol. 8, no. 2, Aug. 1979, pp. 143-5,
https://ictp.acad.ro/jnaat/journal/article/view/1979-vol8-no2-art4
.