Jacob de Gelder (1824). Allereerste Gronden der Cijferkunst [Introduction to Numeracy] (in Dutch). 's-Gravenhage and Amsterdam: de Gebroeders van Cleef. pp. 155–156. Retrieved 2 March 2011.
Ferdinand Malaisé (1842). Theoretisch-practischer Unterricht im Rechnen [Theoretical and practical instruction in arithmetic] (in German). München. pp. 307–322. Retrieved 26 March 2011.
kb.nl
resources2.kb.nl
"[News from] Nederland"(PDF). De Locomotief. Nieuws, handels en Advertentie-blad. 12 August 1869. p. 2. Archived from the original(PDF) on 27 February 2017.
"[News from] Nederland"(PDF). De Locomotief. Nieuws, handels en Advertentie-blad. 12 August 1869. p. 2. Archived from the original(PDF) on 27 February 2017.