The Extended Sum of the Digits of a Natural Number in the Ddecimal Representation "The Test With 9" and Fermat's Diagnosis

Tatiana Dosescu
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Tatiana Dosescu: Dimitrie Cantemir Christian University

Social-Economic Debates, 2019, vol. 8, issue 1, 66-76

Abstract: This note presents a justification of the well-known procedure called "the test with 9" to verify the correctness of some sums or multiplications with natural numbers. The diagnosis of a 12-digit number that was non-prime, was quickly established by Fermat in the 17th century. To verify this result was used "the test with 9".

Keywords: Algebraic structures: group; unitary commutative ring; the grammar of a language; the divisibility criterion with 9 in N; the relation of congruence (search for similar items in EconPapers)
JEL-codes: C12 C14 (search for similar items in EconPapers)
Date: 2019
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