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In number theory, a Carmichael number is a composite number , which in modular arithmetic satisfies the congruence relation: for all integers which are relatively prime to . The relation may also be expressed in the form: . Carmichael numbers are named after American mathematician Robert Carmichael, the term having been introduced by Nicolaas Beeger in 1950 (Øystein Ore had referred to them in 1948 as numbers with the "Fermat property", or "F numbers" for short).They are infinite in number. The Carmichael numbers form the subset K1 of the Knödel numbers.