I subscribe to Algtop-L, an email list that posts algebraic topology news including abstracts of papers, as well as conference announcements. I don't usually expect to understand anything, because I'm so out of it, but I get an idea where people are at and that makes me feel like I'm still part of the community.
And then there are posts like this one. "Deces d'Henri Cartan" it said, the subject of a forwarded email, beginning "La Société Mathématique de France (SMF) a la tristesse d'annoncer le décès d'Henri Cartan survenu le 13 août 2008 à Paris à l'age de 104 ans" and I found it easy to understand even though I've never taken a French course.
Henri Cartan was a mathematician I knew of best as co-author (with Samuel Eilenberg) of a book called Homological Algebra, which another famous French mathematician Jean-Pierre Serre lampooned in a text on algebra by giving as his only exercise in his chapter on homological algebra this: pick up a book on it and prove all the theorems without reading the book's proofs. I don't recall that Serre mentioned Homological Algebra specifically, but since Serre had been a student of Cartan everyone could guess he meant that book.
The proofs may have been trivial but the structure the book revealed was endlessly useful.
It was published in 1956! It was 15 years old when I started using it. So I have been assuming Cartan must have died years ago. Not so. As the email said, he died this month at the age of 104.
Cartan was also one of the creators of Nicolas Bourbaki [pictured left], the invented mathematician who since 1939 has written a series of volumes titled Éléments de mathématique with the lofty goal of unifying mathematics on a set theoretic foundation. Many mathematicians were in fact inspired to unify in mathematical anarchy opposed to what they saw as a Bourbaki imperialism.
Cartan's seminars in the 40s and 50s laid much of the groundwork for all of the topics that algebraic topologists work on today. Good life.