Otto Blumenthal (1935). David Hilbert (ed.). Lebensgeschichte. Gesammelte Abhandlungen. Vol. 3. Julius Springer. pp. 388–429. Archived from the original on 2016-03-04. Retrieved 2019-08-10. Here: p.402-403
Otto Blumenthal (1935). David Hilbert (ed.). Lebensgeschichte. Gesammelte Abhandlungen. Vol. 3. Julius Springer. pp. 388–429. Archived from the original on 2016-03-04. Retrieved 2019-08-10. Here: p.402-403