Robert D. Carmichael (1879-1967) was an American mathematician. Carmichael is known for his research number theory and the study of the prime numbers. Among other discoveries, he identified the first composite number to satisfy the Fermat primality test (561). Such numbers are now called Carmichael numbers in his honor.

Early in his career, he wrote two elementary texts on number theory, "The Theory of Numbers" and "Diophantine Analysis." These cover such topics as prime numbers, quadratic residues, and Fermat's Last Theorem. They require little more than high school algebra, and their...