Steffens, Karl-Georg (2006). Anastassiou, George A (ed.). The history of approximation theory. From Euler to Bernstein. Boston, MA: Birkhäuser Boston, Inc. pp. 111–113. doi:10.1007/0-8176-4475-X. ISBN978-0-8176-4353-9. MR2190312.
doi.org
Steffens, Karl-Georg (2006). Anastassiou, George A (ed.). The history of approximation theory. From Euler to Bernstein. Boston, MA: Birkhäuser Boston, Inc. pp. 111–113. doi:10.1007/0-8176-4475-X. ISBN978-0-8176-4353-9. MR2190312.