Xenakis (Iannis) was a modern Greek composer and architect. For some time he worked full-time for Le Corbusier and only composed as a hobby. He pioneered in writing computer-generated music, which is to say, he wrote the mathematical algorithms and on which the computer based the music. He also published essays on his findings about the mathematical properties of music.
And you thought I was going to skip the letter X, didn't you?