Tangent sheaf
in algebraic geometry
The sheaf on an algebraic variety or scheme X over a field k , whose sections over an open affine subspace U = \mathop{\rm Spec} ( A) are the A - modules of k - derivations \mathop{\rm Der} _ {k} ( A, A) of the ring A . An equivalent definition is that \theta _ {X} be the sheaf of homomorphisms \mathop{\rm Hom} ( \Omega _ {X/k} ^ {1} , {\mathcal O} _ {X} ) of the sheaf of differentials \Omega _ {X/k} ^ {1} into the structure sheaf {\mathcal O} _ {X} ( see Derivations, module of).
For any rational k - point x \in X , the stalk \theta _ {X} ( x) of the sheaf \theta _ {X} is identical to the Zariski tangent space T _ {K,x} to X at x , that is, to the vector k - space \mathop{\rm Hom} _ {k} ( \mathfrak M _ {x} / \mathfrak M _ {x} ^ {2} , k) , where \mathfrak M _ {x} is the maximal ideal of the local ring {\mathcal O} _ {K,x} . Instead of the tangent sheaf \theta _ {X} one can use the sheaf of germs of sections of the vector bundle V ( \Omega _ {X/k} ^ {1} ) dual to \Omega _ {X} ^ {1} ( or the tangent bundle to X ). In the case when X is a smooth connected k - scheme, \theta _ {X} is a locally free sheaf on X of rank equal to the dimension of X .
References
[1] | I.R. Shafarevich, "Basic algebraic geometry" , Springer (1977) (Translated from Russian) MR0447223 Zbl 0362.14001 |
[a1] | R. Hartshorne, "Algebraic geometry" , Springer (1977) pp. Sect. IV.2 MR0463157 Zbl 0367.14001 |
Tangent sheaf. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Tangent_sheaf&oldid=53762