An algorithm for computing the GCD of two numbers was described in the ancient Chinese mathematics book The Nine Chapters on the Mathematical Art. The original algorithm was used to reduce a fraction. The description reads:
"If possible halve it; otherwise, take the denominator and the numerator, subtract the lesser from the greater, and do that alternately to make them the same. Reduce by the same number."
This just looks like a normal Euclidian algorithm, but the ambiguity lies in the phrase "if possible halve it". The traditional interpretation is that this only works when 'both' numbers to begin with are even, under which the algorithm is just a slightly inferior Euclidean algorithm (for using subtraction instead of division). But the phrase may as well mean dividing by 2 should 'either' of the numbers become even, in which case it is the binary GCD algorithm.
Read more about this topic: Binary GCD Algorithm
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