Erdős number
![Paul Erdős in 1992](/Images/godic/202501/10/Erdos_budapest_fall_19925434.jpg")
![Paul Erdős teaching Terence Tao in 1985 at the University of Adelaide. Tao, who was 10 years old at the time, became a professional mathematician. He received the Fields Medal in 2006 and was elected a Fellow of the Royal Society in 2007. Tao has an Erdős number of 2](/Images/godic/202501/10/Paul_Erdos_with_Terence_Tao5434.jpg")
The Erdős number (Hungarian pronunciation: [ˈɛrdøːʃ]) describes the "collaborative distance" between mathematician Paul Erdős and another person, as measured by authorship of mathematical papers.
The same principle has been applied in other fields where a particular individual has collaborated with a large and broad number of peers. The American Mathematical Society provides a free online tool to determine the Erdős number of every mathematical author listed in the Mathematical Reviews catalogue.