MathOverflow is a question and answer site for professional mathematicians. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Question: Let $n$ be a positive integer and let $X$ be a simplicial toric variety. Does every coset of $n\cdot Pic(X)\subseteq Pic(X)$ contain a base point free linear system?

If $X$ is not simplicial, the answer is definitely no, since there exist proper toric varieties with no non-trivial line bundles (for example, Example 4.2.13 of Cox, Little, and Schenck).

Obviously, if $X$ is quasi-projective, every coset of $n\cdot Pic(X)$ contains a very ample (so base point free) line bundle. So the question is only interesting for non-quasi-projective $X$.

I'll rephrase the question in terms of fans. Given a rational ray $\rho$, let $v_\rho$ denote the first lattice point along $\rho$.

Rephrasing: Let $n$ be a positive integer and let $\Sigma$ be a simplicial fan on a lattice $N$. Let $\{a_\rho\}_{\rho\in \Sigma(1)}$ be integers associated to the rays of $\Sigma$. Do there exist integers $\{b_\rho\}_{\rho\in \Sigma(1)}$ so that for every $\rho$, there is a point $\chi\in N^*$ so that $\chi(v_\rho) + a_\rho+nb_\rho=0$ and $\chi(v_{\rho'})+a_{\rho'}+nb_{\rho'}\ge 0$ for every $\rho'\in \Sigma(1)$?

These sets of integers are assumed to be induced by integer-valued piecewise linear functions. That is, they correspond to Cartier divisors.

In other words, given a bunch of integral hyperplanes perpendicular to the rays of $\Sigma$, is it possible to move each of them by a multiple of $n$ (, sigh) so that each of them touches the polytope they define?

share|cite|improve this question

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.