A subset is algebraically independent over if every element of is transcendental over and over each of the extension fields over generated by the remaining elements of .
1999, David Mumford, The Red Book of Varieties and Schemes: Includes the Michigan Lectures, Springer, Lecture Notes in Mathematics 1358, 2nd Edition, Expanded, page 40,
If thestatementisfalse, there are elements in such that their images in are algebraically independent. Let . Then cannot be algebraically independent over , so there is a polynomial over such that .
2006, Alexander B. Levin, Difference algebra, M. Hazewinkel (editor), Handbook of Algebra, Volume 4, Elsevier (North-Holland), page 251,
Setting (where 1 denotes the identity of the semigroup ) we obtain a -algebraically independent over set such that .