The first picture, from the private collection of Zermelo's wife Gertrud (1902-2003), shows Zermelo in 1907, when he worked in Göttingen in David Hilbert's group.

On the second picture we see a draft to his article Beweis, daß jede Menge wohlgeordnet werden kann, Mathematische Annalen 1904, Volume 59, Issue 4, pp. 514-516, in which he proved the well-ordering theorem. In the displayed passage the axiom of choice is mentioned. Zermelo worked in Göttingen from 1897 - 1910. Now parts of his inheritance are in the Freiburg university archive.

The third picture shows Zermelo in his Zürich time, 1910 - 1916.

From 1926 - 1935 and from 1946 - 1953 Zermelo was a honorary full professor at the Mathematical Institute of the Albert-Ludwigs-Universität Freiburg. During his Freiburg time, Zermelo worked mainly in analysis and applied mathematics, but also in set theory, establishing for example, the most important axiom system ZFC for set theory. In 1935 Zermelo renounced his honorary professorship at the University of Freiburg after he was denounced for refusing the Hitler salute. The fourth picture shows his letter of renunciation. He was reinstalled in 1946. You can press control plus to enlarge the letter and read the mordant words.

We thank Heinz-Dieter Ebbinghaus for sharing his work on the Zermelo biography with us. We are grateful to Thomas Lais who processed the pictures and the archive material and generously loaned his work to us. Thanks also go to the Universitätsarchiv Zürich and the Universitätsarchiv Freiburg for their permission to use the pictures.

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