|From the article: Erdos with Terrence Tao in 1985.|
"In 1984, when he won the prestigious Wolf Prize in Mathematics, he used nearly all of the $50,000 award to endow a prize in memory of his parents, which is presented by the Israel Mathematical Union.
"Although Erdos lacked many social graces, he had an innocence and generosity that brought him countless friends and collaborators – more than 500 different people co-authored papers with him. Hence arose the concept of an “Erdos number,” by which people denote the degree of separation between them and Erdos, beginning with one degree for those who actually worked directly with him. Some 200,000 mathematicians are said to have assigned themselves Erdos numbers." -- from the article.