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Euclid's Number-Theoretical Work

When people mention the mathematical achievements of Euclid, his geometrical achievements always spring to mind. But, his Number-Theoretical achievements (See Books 7, 8 and 9 in his magnum opus \emph{Elements} [1]) are rarely spoken. The object of this paper is to affirm the number-theoretical role of Euclid and the historical significance of Euclid's algorithm. It is known that almost all elementary number-theoretical texts begin with Division algorithm. However, Euclid did not do like this. He began his number-theoretical work by introducing his algorithm. We were quite surprised when we began to read the \emph{Elements} for the first time. Nevertheless, one can prove that Euclid's algorithm is essentially equivalent with the Bezout's equation and Division algorithm. Therefore, Euclid has preliminarily established Theory of Divisibility and the greatest common divisor. This is the foundation of Number Theory. After more than 2000 years, by creatively introducing the notion of congruence, Gauss published his \emph{Disquisitiones Arithmeticae} in 1801 and developed Number Theory as a systematic science. Note also that Euclid's algorithm implies Euclid's first theorem (which is the heart of `the uniqueness part' of the fundamental theorem of arithmetic) and Euclid's second theorem (which states that there are infinitely many primes). Thus, in the nature of things, Euclid's algorithm is the most important number-theoretical work of Euclid. For this reason, we further summarize briefly the influence of Euclid's algorithm. Knuth said `we might call Euclid's method the granddaddy of all algorithms'. Based on our discussion and analysis, it leads to the conclusion Euclid's algorithm is the greatest number-theoretical achievement of the Euclidean age.