If you need a letter of recommendation from Prof. Dr. Bent Gebert or Dr. Nico Kunkel, it is important that you are known to the reviewer by having attended at least one course.
Please contact them by email in a timely manner (at least three weeks before the submission deadline) to discuss your request. Please also have relevant documents (esp. letter of motivation, transcript of records/grading survey, curriculum vitae, references from the foundation/institution) at hand.