Notational Question (a,x)

[Toni]

Up above there is a picture from "A VERY ELEMENTARY PROOF THAT THE SOMOS 5 SEQUENCE IS INTEGER VALUED" (Name of the paper). Now I'm having problems with the notation (a,x) from Lemma 2.1 & 2.2. I do not understand what it means. I tried vectors but this doesn't make sense to me since it is equal to 1, and open intervals wich didn't make sense either. Does someone have an idea what this notation is about?

 

[Steven G]

(r,s) is shorthand notation for gcd(r,s), ie the greatest common divisor of r and s

 

[Toni]

Thank you very much, you have indeed saved me. Have a great day!