When I arrived at Columbia, I had some training in knot theory. Walter opened my eyes to the breadth and depth of geometry. But I learned so much more from Walter. Here is a partial top 10 list of Walter's lessons:

1. Have a definite problem in mind. Otherwise, you're wasting your time.

2. Find the right special case. The essential features and difficulties of your problem will be revealed, and you can start making progress.

3. Mathematics is unified. Learn any math you need to solve your problem. Be fearless. Walter's own fearlessness made it look easy to learn new areas.

4. Use a computer to experiment. It helps to acquire the right intuition. But then work on paper.

5. Lunch with the speaker is essential. Invite and subsidize graduate students to build a community around the seminar. Order the bibimbap.

6. Always ask a question. Our seminars were always very active, but if nobody had a question, then Walter always had a wonderful question to ask at the end of the talk.

7. Write with verve. Walter is a wonderful writer. He taught me how to write papers, NSF grants, referee reports, and how to write difficult letters to prominent mathematicians when we co-edited a book. I learned that being a good editor requires political savvy.

8. Write thoughtful reference letters. Walter worked for many hours on countless reference letters for mathematicians from all over the world. His letters were invaluable for my own career, but I saw the care he took with letters for people he had never met.

9. Do math while hiking. My favorite math conversations occurred while hiking with Walter at Oberwolfach, Luminy, Banff, and many other conferences. You have to walk fast to keep up, but it’s worth it!

10. Don't erase the board. Someone using the classroom after our seminar complained that our board had not been erased. Walter immediately responded that he loved seeing a full board when he entered a room! He liked to figure out what had been discussed in the previous seminar. When you love mathematics as Walter does, a mysterious full board is not a chore, but an invitation to ponder.

Thank you, Walter, for these lessons. They have made me a better mathematician and a better person, and I am grateful for all your kindness.

—Ilya Kofman