For Paul Erdös, mathematics was life. Number theory, combinatorics (a branch of mathematics concerning the arrangement of finite sets), and discrete mathematics were his consuming passions. Everything else was of no interest: property, money, clothes, intimate relationships, social pleasantries--all were looked on as encumbrances to his mathematical pursuits. A genius in the true sense of the word, Erdös traveled the world, living out of a suitcase, to problem solve--and problem pose--with his mathematical peers. A small, hyperactive man, he would arrive at a university or research center confident of his welcome. While he was their guest, it was a host's task to lodge him, feed him, do his laundry, make sure he caught his plane to the next meeting, and sometimes even do his income taxes. Cosseted by his mother and by household servants, he was not brought up to fend for himself. Gina...