determine u(R[ x] )?

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Are you wanting to find the unit group of an arbitrary polynomial ring? That is more of a mathematical question than a Sage question. If R is commutative then the units in R[X] are the polynomials with unit constant coefficient and all other coefficients nilpotent (exercise), so if R is an integral domain you just get the constant units.

What exactly would you expect from Sage here?

If your question is different, please explain more carefully!