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Suppose that $S_1$ and $S_2$ are the vanishing sets of a system of polynomial equations in n variables over a field $\mathbb{k}$ (ideal in $\mathbb{k}[X_1,\dots,X_n]$) and a system of polynomial equations in m variables over a field $\mathbb{k}$ (ideal in $\mathbb{k}[Y_1,\dots,Y_m]$). We can give $S_1$ and $S_2$ the Zariski topology by letting all algebraic subsets of $S_1$ and $S_2$ to be closed.

Surely, if $Y_j\circ f\colon S_1\to\mathbb{k}$ is a polynomial in $\mathbb{k}[X_1,\dots,X_n]/I(S_1)$, for every $j$, then $f$ is continuous in the Zariski topology.

The condition that $Y_j\circ f$ be polynomial, however, seems too strict: as long as the vanishing set of $Y_j\circ f$ were equal to the vanishing set of a polynomial function, $f$ would be continuous, which means that the usual morphisms from $S_1$ to $S_2$ are not the same as the continuous maps between them as (Zariski-)topological spaces.

My question then is of two parts.

  1. Is it true that there exist Zarsiki-continuous maps that are not polynomial?
  2. If yes, then what is the geometric way of thinking about the extra structure that polynomial maps preserve, i.e. continuous maps squash subsets of $S_1$ in such a way that non-algebraic subsets never get squashed into algebraic ones, but some algebraic ones may get squashed into non-algebraic ones, but polynomials also give what features to the geometric squashing?
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4 Answers

up vote 14 down vote accepted

If I understand your question correctly, then (1) yes, there are going to be lots of Zariski continuous maps that are not polynomial (although, if one asks that all higher products of the map preserve the Zariski topology on the corresponding sources and targets, this is a much stronger restriction, I think --- does anyone know the details?); and (2) the usual extra structure that one adds is the sheaf of regular functions on source and target; polynomial maps pullback regular functions on the target to regular functions on the source. Thus varieties are naturally thought of as being objects in the category of locally ringed spaces (i.e. spaces equipped with a certain structure sheaf of rings) rather than just in the category of topological spaces.

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Consider the affine line and Look for an example there yourself.

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Just an easy remark. If $k = \mathbb{F}_q$ is the finite field with $q$ elements, then every function $k^n \to k$ is a polynomial map. In fact, for every point $p = (a_1, \dots, a_n) \in k^n$ consider the polynomial $$ f(x_1, \dots, x_n) = (-1)^n \prod_{i=1}^n \prod_{b \in \mathbb{F}_q \setminus \{ a_i \} } (x_i - b); $$ $f(p) = 1$ and $f$ is zero in $k^n \setminus \{ p \}$.

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Yes, there exists Zariski continuous maps between algebraic varieties that are not polynomials. For example all bijection between $\mathbb A ^1$ and itself is Zariski continuous but not polynomial in general: think of bijection that permutes $0$ and $1$ and leaves other points fixed.

Geometric way of thinking is algebraic subsets must be preserved. Only use polynomials and rational functions.

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-1; the image of an algebraic set under a polynomial map need not be algebraic. –  Qiaochu Yuan Nov 7 '10 at 21:57
@Qiaochu: Dear Qiaochu, As you know, though, the image is constructible (in the Zariski topology), which is pretty close! –  Matt E Nov 7 '10 at 22:03
Thank you Matt E: you are good person . I wanted give intuitive explanation. I know Chevalley theorem : given morphism of finite presentation between schemes and constructible subset above, image is constructible! But I had not intention to comment: negative votes are joy to give but also to take! –  evgeniamerkulova Nov 7 '10 at 23:01
@Matt E: do you know if this condition characterizes polynomial maps among Zariski-continuous maps? –  Qiaochu Yuan Nov 8 '10 at 0:40
@Qiaochu Yuan: Dear Qiaochu, I don't think so; if one thinks about maps between curves, this doesn't seem to add much of a restriction (because the topology is determined just by cardinality considerations, so any self-bijection of the curve will be Zariski continuous and preserve constructibility). If you look at products as well, then I think maybe the situation changes ... . –  Matt E Nov 8 '10 at 1:38
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