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NormalToricVarieties :: isCartier(ToricDivisor)

isCartier(ToricDivisor) -- whether a torus-invariant Weil divisor is Cartier

Synopsis

Description

A torus-invariant Weil divisor $D$ on a normal toric variety $X$ is Cartier if it is locally principal, meaning that $X$ has an open cover $\{U_i\}$ such that $D|_{U_i}$ is principal in $U_i$ for every $i$.

On a smooth variety, every Weil divisor is Cartier.

i1 : PP3 = toricProjectiveSpace 3;
i2 : assert all (3, i -> isCartier PP3_i)

On a simplicial toric variety, every torus-invariant Weil divisor is $\QQ$-Cartier, which means that every torus-invariant Weil divisor has a positive integer multiple that is Cartier.

i3 : W = weightedProjectiveSpace {2,5,7};
i4 : assert isSimplicial W
i5 : assert not isCartier W_0
i6 : assert isQQCartier W_0
i7 : assert isCartier (35*W_0)

In general, the Cartier divisors are only a subgroup of the Weil divisors.

i8 : X = normalToricVariety (id_(ZZ^3) | -id_(ZZ^3));
i9 : assert not isCartier X_0
i10 : assert not isQQCartier X_0
i11 : K = toricDivisor X;

o11 : ToricDivisor on X
i12 : assert isCartier K

See also