Length 2 variables of $A[x,y]$ and transfer
We construct and study length $2$ variables of $A[x,y]$ ($A$ is a commutative ring). If $A$ is an integral domain, we determine among these variables those which are tame. If $A$ is a UFD, we prove that these variables are all stably tame. We apply this construction to show that some polynomials of $A[x_1,\dots,x_n]$ are variables using transfer.