Potra, F.-A. “A Characterisation of the Divided Differences of an Operator Which Can Be Represented by Riemann Integrals”. Anal. Numér. Théor. Approx., vol. 9, no. 2, Aug. 1980, pp. 251-3, https://ictp.acad.ro/jnaat/journal/article/view/1980-vol9-no2-art10.