definition of gcd for(x,y)

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I'm not clear on exactly what your question is, but here is some information.

The syntax for gcd is:

gcd(a,b)

You can get help on the gcd command by typing gcd? at the prompt. If you want to see the details of Sage's gcd code, you can type gcd?? at the prompt.

Hi, you can always define your own gcd function with the properties you need. There are various equivalent definitions of gcd as these websites show: Paper, Wolfram and Wiki Roland