Suppose that p and q are naturals such that gcd(p,q)=1. Let N∈N be arbitrary and suppose that gcd(p+k1N,q)>1 for some k1∈Z. Does there exist k2∈Z such that gcd(p+k1N,q+k2N)=1?
I strongly suspect that this is true, after I ran several computer verifications. However, I can't seem to prove this, nor construct a counterexample. Here's what I've tried though:
Let A:={a1,a2,...,an} be the prime factors of p, B:={b1,b2,...,bm} the prime factors of q and C:={c1,c2,...,cl} the prime factors of p+k1N. Then if we list the common prime factors between p+k1N and q as {ci,ci+1,...,ci+j}, then because they all divide p+k1N and none of them divides p (otherwise gcd(p,q)>1), we must have that none of them divides k1N so, in particular, none of them divides N. Thus gcd(p+k1N,q+N)≠gcd(p+k1N,q)≠1.
I hope to continue in this fashion the process of removal of common factors by adding clever multiples of N to q until we end up removing all of them, but I can't see how.
I don't know if this is a known result or not since I couldn't find it anywhere (although it looks very elementary).
Thank you in advance!
Answer
By here if gcd(a,b,c)=1 then there exists k2 such that gcd(Nk2 + q,Nk1+p) ⏞gcd(ak2+b,c)=1, true for OP by gcd(a,b,c)=gcd(N,q,Nk1+p)=gcd(N,q,p)=1, by gcd(p,q)=1
As explained there, a solution can be efficiently computed by gcd calculations.
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