https://www.mappingthelives.org/bio/609a904c-ef82-44d0-a694-4f30e60b8f81
https://www.joodsmonument.nl/en/page/149986/gustav-heijmann
https://www.bundesarchiv.de/gedenkbuch/en855495
Gustav Heymann (1867 - 1943) - Genealogy (geni.com)