Celebratio Mathematica

I met Tom Lig­gett in the aca­dem­ic year 2015–2016, when I was an ad­junct as­sist­ant pro­fess­or at UCLA. But be­fore that I was for­tu­nate enough to have writ­ten a pa­per with him and my Ph.D. ad­visor, Rick Dur­rett, on the con­tact pro­cess with fast vot­ing. I can still re­mem­ber how I was totally amazed by his mas­tery and ma­gic in dis­crete har­mon­ic ana­lys­is. Tom had of­fi­cially re­tired be­fore I went to UCLA, but still he was the first per­son who took me to the beau­ti­ful fac­ulty club (and showed me how to re­gister there)! We met roughly every one or two weeks when I was at UCLA, and I was deeply touched by his lifelong fo­cus and de­vo­tion to dis­crete prob­ab­il­ity.

In ad­di­tion to his repu­ta­tion as a world-class math­em­atician, Tom was also known as a won­der­ful math­em­at­ic­al edu­cat­or. Early in 2016, I brought him my un­der­gradu­ate prob­ab­il­ity teach­ing eval­u­ations for his feed­back. Tom told me the ma­gic for­mula he used to help people im­prove their teach­ing when he served as Dir­ect­or of Un­der­gradu­ate Stud­ies at the math de­part­ment of UCLA. He ad­vised me to take a closer look at the com­ments and find out what the com­plaints have in com­mon. With this fo­cus we quickly found that I needed a more care­ful design to my black­board-writ­ing, and that I needed to avoid con­stant eras­ing of re­cent ma­ter­i­al. I still find his re­com­mend­a­tion to be the most prac­tic­al guideline for im­prov­ing my teach­ing.

After leav­ing UCLA, I fre­quently sent him up­dates about my re­search and ca­reer. Tom’s replies were al­ways warm, en­cour­aging, and full of wis­dom. To­wards the be­gin­ning of 2019, I still re­ceived re­marks from him with great in­sight re­gard­ing the threshold-one con­tact pro­cess/voter mod­els. At some point, I began (wrongly) to re­call that our talks were just on math­em­at­ics. But when I star­ted pre­par­ing this trib­ute, all the stor­ies he shared with me about his life ree­m­erged mi­ra­cu­lously: those of his early life in the Mid­w­est and Ar­gen­tina, those of LA air pol­lu­tion in the old days, and those of his evac­u­ation in a wild­fire by the Na­tion­al Park Rangers.

I am so deeply sor­rowed by Tom’s passing away. It is an hon­or to have re­ceived his pre­cious help and ad­vice in re­search, teach­ing, and life.

Yuan Zhang is As­sist­ant Pro­fess­or at Pek­ing Uni­versity's School of Math­em­at­ic­al Sci­ences in Pek­ing, China.