One of my math professors (in a U.S. university) retired this semester. He was one person that I had planned on asking to write my letters of recommendation this coming fall semester, when I send out PhD applications in math / applied math.
He is very old and he seems anti-social and seems to not communicate with our math department very much. I am just basing this on some stuff that he says, which scares me, e.g., "tell them (our math dept.) that I said this..."
So, I wonder whether anyone external to our own math department would even know who he is - or whether admissions committees even value his opinions about grad students, at this point in his career.
Would it be best to find someone else to replace him as one of my letter writers? Someone who is younger and currently active in research? I do have replacements in mind, but I am somewhat attached to this particular professor, since I feel that I got my start in rigorous mathematics with him.
Edit: To add a bit more context, he is a full professor and has served on admissions / doctoral committees at our department numerous times. So, he's very much qualified, of course. My main concern is that perhaps he no longer communicates actively with people internal and external to our department, but that is only my best guess.