Gauss lemma (content multiplicativity)
In a UFD, the content of a product equals the product of contents up to associates.
Gauss content lemma: Let be a UFD. For in the polynomial ring, let denote the content of (a gcd of its coefficients, defined up to units). Then
where denotes equality up to associates. Equivalently, the product of two primitive polynomials is primitive.
Remarks
This lemma is the technical engine behind Gauss-type transfer results between and .