Birational equivalence is a key concept in algebraic geometry, comparing varieties that are "almost isomorphic." It allows for differences in lower-dimensional subsets, making it weaker than but still preserving important geometric properties.
Understanding birational equivalence helps us study varieties by relating them to simpler ones. It's crucial for classification problems and gives insights into the structure of algebraic varieties, connecting to function fields and rational maps in meaningful ways.
Birational Equivalence of Varieties
Definition and Implications
Top images from around the web for Definition and Implications
ag.algebraic geometry - Names of certain surfaces - MathOverflow View original
Is this image relevant?
algebraic geometry - Different definitions of rational mappings. - Mathematics Stack Exchange View original
Is this image relevant?
algebraic geometry - Different definitions of rational mappings. - Mathematics Stack Exchange View original
Is this image relevant?
ag.algebraic geometry - Names of certain surfaces - MathOverflow View original
Is this image relevant?
algebraic geometry - Different definitions of rational mappings. - Mathematics Stack Exchange View original
Is this image relevant?
1 of 3
Top images from around the web for Definition and Implications
ag.algebraic geometry - Names of certain surfaces - MathOverflow View original
Is this image relevant?
algebraic geometry - Different definitions of rational mappings. - Mathematics Stack Exchange View original
Is this image relevant?
algebraic geometry - Different definitions of rational mappings. - Mathematics Stack Exchange View original
Is this image relevant?
ag.algebraic geometry - Names of certain surfaces - MathOverflow View original
Is this image relevant?
algebraic geometry - Different definitions of rational mappings. - Mathematics Stack Exchange View original
Is this image relevant?
1 of 3
Two algebraic varieties X and Y are if there exist rational maps f:X→Y and g:Y→X such that g∘f=idX and f∘g=idY, where idX and idY are the identity maps on X and Y, respectively
Birational equivalence captures the idea that two varieties are "almost isomorphic," meaning they are isomorphic except on lower-dimensional subsets (singular points, exceptional curves)
The dimension of birationally equivalent varieties must be the same
Birational equivalence is a weaker notion than isomorphism, as it allows for the varieties to differ on subsets of lower dimension
Function Fields and Birational Equivalence
The function fields of birationally equivalent varieties are isomorphic
If X and Y are birationally equivalent, then K(X)≅K(Y), where K(X) and K(Y) are the function fields of X and Y, respectively
The isomorphism between function fields induced by birational equivalence preserves the field operations and the transcendence degree
Conversely, if the function fields of two varieties are isomorphic, then the varieties are birationally equivalent (Zariski's theorem)
Birational Equivalence as an Equivalence Relation
Reflexivity
For any algebraic variety X, the idX:X→X is a , and idX∘idX=idX, showing that X is birationally equivalent to itself
The reflexive property holds trivially for any variety, as the identity map is always a birational equivalence
Symmetry
If X and Y are birationally equivalent with rational maps f:X→Y and g:Y→X satisfying g∘f=idX and f∘g=idY, then Y and X are also birationally equivalent
The same maps f and g demonstrate the birational equivalence in the opposite direction
Symmetry follows from the definition of birational equivalence, as the roles of X and Y can be interchanged
Transitivity
If X and Y are birationally equivalent with rational maps f:X→Y and g:Y→X, and Y and Z are birationally equivalent with rational maps h:Y→Z and k:Z→Y, then X and Z are birationally equivalent
The composition of birational maps h∘f:X→Z and g∘k:Z→X demonstrates the birational equivalence between X and Z
Transitivity allows for the "chaining" of birational equivalences to establish equivalence between varieties not directly related by a single pair of birational maps
Constructing Birational Maps
Finding Birational Maps
To construct a birational map between two algebraic varieties X and Y, find rational maps f:X→Y and g:Y→X such that g∘f and f∘g are the identity maps on X and Y, respectively, outside of lower-dimensional subsets
The maps f and g may not be defined everywhere on the varieties, but they should be defined on open dense subsets
Techniques for constructing birational maps include using projections (projecting from a point or a subvariety), blowups (resolving singularities or indeterminacies), and blowdowns (contracting subvarieties)
Inverses of Birational Maps
The inverse of a birational map f:X→Y is a rational map g:Y→X such that g∘f=idX and f∘g=idY, where idX and idY are the identity maps on X and Y, respectively
Birational maps and their inverses may not be defined everywhere on the varieties, but they are defined on open dense subsets
To find the inverse of a birational map, one can solve for the pre-image of a general point under the map and express the result as a rational function (e.g., using coordinate functions)
The inverse of a birational map is unique up to equality on an open dense subset
Birational Equivalence vs Isomorphism
Comparing the Notions
Isomorphism is a stronger notion than birational equivalence, as it requires the existence of morphisms (regular maps) f:X→Y and g:Y→X such that g∘f=idX and f∘g=idY everywhere on the varieties
Birationally equivalent varieties may have different local structures, such as singularities or different numbers of irreducible components, while isomorphic varieties have the same local structure
Isomorphic varieties have the same dimension and degree, while birationally equivalent varieties only need to have the same dimension
Every isomorphism is a birational equivalence, but not every birational equivalence is an isomorphism
Examples Illustrating the Difference
The projective line P1 and the affine line A1 are birationally equivalent but not isomorphic
The map f:A1→P1 given by x↦[x:1] and its inverse g:P1→A1 given by [x:y]↦x/y (for y=0) demonstrate the birational equivalence
However, there is no isomorphism between P1 and A1 because they have different global structures (P1 is compact, while A1 is not)
A singular cubic curve and a non-singular cubic curve are birationally equivalent but not isomorphic
The normalization map from the non-singular curve to the singular curve is a birational equivalence
The curves are not isomorphic because they have different local structures at the singular point