Lothar Collatz
Lothar Collatz (; 6 July, 1910 – 26 September 1990) was a German mathematician. He was born in Arnsberg, Westphalia. The "3x + 1" problem also known as the Collatz conjecture was named after him, and is still not solved. The Collatz–Wielandt formula for the Perron–Frobenius theorem was also named after him. His 1957 paper with Ulrich Sinogowitz, who was killed in the bombing of Darmstadt in World War II, started the field of spectral graph theory.