Abstract
We extend Alon’s Combinatorial Nullstellensatz from polynomial rings over fields to polynomial rings over division rings and to rings of polynomial functions over centrally finite division algebras. We apply our results to extend classical theorems from additive number theory to the additive theory of division rings, where the size of algebraic sets is measured by their rank in the sense of Lam.
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Notes
Variations of these results appeared in earlier works of Alon and his colleagues, but [2] is the seminal paper on the subject.
We denote the standard non-real generators of \({\mathbb {H}}\) by \({\textrm{i}}, {\textrm{j}},{\textrm{k}}\), as opposed to the letters i, j, k which we reserve for indices.
Recall that \(b^{b-a}\) denotes the conjugation \((b-a)b(b-a)^{-1}\) of b by \(b-a\).
If K is of characteristic 0, then this condition is void.
We note that in the literature there are alternative notations for these rings—for example in [8], for \(n=1\), the ring of polynomials in a central variable x over D is denoted by \(D_L[x]\), while the ring of polynomial functions in the variable x over D is denoted by \(D_G[x]\).
This stands in stark contrast to the case of finite field extensions. For example, consider the linear polynomial \(z \in {\mathbb {C}}[z]\). Its real and imaginary components are of course not polynomial functions in z.
Note that here substitution at a point in \(D^n\) is a homomorphism—unlike the situation considered in the previous sections.
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Acknowledgements
The author wishes to thank Francois Legrand and Shoni Gilboa for useful conversations on the topic of this paper, and the anonymous referee, for his/her careful reading and many comments which helped improve the presentation of this work.
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Paran, E. Combinatorial Nullstellensatz over division rings. J Algebr Comb 58, 895–911 (2023). https://doi.org/10.1007/s10801-023-01263-1
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DOI: https://doi.org/10.1007/s10801-023-01263-1