Lothar Collatz

Lothar Collatz (; July 6, 1910 – September 26, 1990) was a German mathematician, born in Arnsberg, Westphalia.

The "3''x'' + 1" problem is also known as the Collatz conjecture, named after him and still unsolved. The Collatz–Wielandt formula for the Perron–Frobenius eigenvalue of a positive square matrix was also named after him.

Collatz's 1957 paper with Ulrich Sinogowitz, who had been killed in the bombing of Darmstadt in World War II, founded the field of spectral graph theory. Provided by Wikipedia