http://nl.wikisage.org/wiki/Ludwig_Falkenstein
https://books.google.be/books/about/Ludwig_Falkenstein.html?id=zZSgAAAAMAAJ&redir_esc=y
http://www.yveslouis.com/files/publicaties/AR1419_012-013.pdf
https://www.mappingthelives.org/bio/5a3e0021-2ce6-4f28-bae9-8ff91a8d4a7c