https://digital.katalog.queersearch.org/s/katalog/item/40731, “Ulrichs: The Life and Works of Karl Heinrich Ulrichs: Pioneer of the Modern Gay Movement”, https://digital.katalog.queersearch.org/s/katalog/item/49635, QueerSearch, accessed 29. April 2025, https://digital.katalog.queersearch.org/s/katalog/item/25973