1. http://www.artindex.nl/noordholland/default.asp?id=6#=0771900359003090113150157001890990501991
2. https://rkd.nl/nl/explore/artists/77552
3. https://www.kunstveiling.nl/kunstenaar/karel-timmerman/5lOZHSjXt6UyyIcOSKAGs8
4. https://www.kunstveiling.be/items/karel-timmerman-1933-2-x-acrylverf-op-papier-jaren-70-ongesigneerd/134389
5. PK A074
5. Familiearchief
The data shown has no sources.