I spent a semester online at Brandeis and while I had never person spoken to Prof. Bellaïche, I have heard about his importance to the department several times during my attendance. He was relatively young and was known to be knowledgable and helpful. I still have the “French for Mathematicians” presentation saved, from his personal website, to benefit from when I go to grad school. My heart goes out to the math dept, his presence will be missed.