Injective map on Coordinate rings implies dense image? Let $V$ and $W$ be (irreducible algebraic) varieties over an algebraically closed field $k$.
Recall $X\subset W$ is dense if and only if $V(I(X))=W$.
Let $f:V\rightarrow W$ be a morphism.
If the pullback $f^\ast\colon k[W]\rightarrow k[V]$ is injective then is the image of $f$ dense in $W$ ?


Prop: Let $F:V\rightarrow W$ be a morphism of affine algebraic varieties then pullback $F^{*}:\mathbb{C}[W]\rightarrow \mathbb{C}[V]$ is injective if
  and only if $F$ is dominant, that is, the image set $F(V)$ is
  dense in $W$.

Proof:
First, let us assume that the image of $V$ under $F$ is dense in $W$.  If $g\in\mathbb{C}[W]$ is in the kernel of $F^{*}$ then by definition $F^{*}(g)=0$ meaning that $g\circ F$ is equivalent zero meaning that $g$ is zero on $F(V)$.  Since $g$ is a polynomial and hence is continous this means it is zero on $W$ as it zero on $F(V)$, which is a dense set.  Thus, the kernel of $F^{*}$ is trivial implying that $F^{*}$ is injective.
Alternatively if we assume $F^{*}$ is injective then to show $F$ is dominate we need to show that if $U\subset W$ is a nonempty open subset of W then $F(V)\cap U$ is nonempty.  In the Zarski topology an open set $U$ is of the form $U=W\setminus X\cap W$ where $X$ is an affine algebraic variety.  However, we know that every affine algebraic variety is equal to the intersection of a finite number of hypersurfaces, and thus it suffices to consider open sets of the form $U=W\setminus \mathbb{V}((f))\cap W$ where $f$ is a complex polynomial.  Now since $U$ is nonempty $f$ is not equivalently zero on $W$ as if it were then $W\subset \mathbb{V}((f))$ implying $U=\emptyset$.  Sine $F^{*}$ is injective this means it has trivial kernel and hence that $F^{*}(f)\not\equiv0$.  By definition this means that $f\circ F\neq0$, and so there exists a $v\in V$ such that $f\circ F(v)\neq0$.  However, by construction $f$ vanishes on precisely $W\setminus U$ and thus $F(v)$ must be in $U$.  This means that $F(V)\cap U\neq\emptyset$ and thus the image of $V$ is dense in $W$.  So we can conclude that the pullback $F^{*}:\mathbb{C}[W]\rightarrow\mathbb{C}[V]$ is injective if and only if $F$ is dominant.
