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Dimensions, Tangent Spaces and Smooth Affine Varieties
Ryan, Lok-Wing Pang
Department of MathematicsHong Kong University of Science and Technology
Introduction to Algebraic Geometry, 2015
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 1
/ 17
Outline
1 Dimension of an Affine Variety
2 Tangent Spaces
3 Smooth Varieties: Motivations
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 2
/ 17
Dimension of an Affine Variety
Throughout the presentation, we assume the underlying field k to bealgebraically closed.
The idea to define dimension in algebraic topology using Zariskitopology is the following:
If X is an irreducible topological space, then any proper closed subsetof X must have dimension (at least) one smaller.
Surprisingly, it turns out that the right definition is purely topological:it depends only on the topological space.
Defintion (Dimension)
Let X be a topological space. Then the dimension dimX of X is definedto be the supremum of all integers n such that there exists a chain ofirreducible closed subsets of X :
X0 ( X1 ( · · · ( Xn
contained in X . In particular, if X is an affine variety, then we define thedimension of the variety to be its dimension as a topological space.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 3
/ 17
Dimension of an Affine Variety
Throughout the presentation, we assume the underlying field k to bealgebraically closed.
The idea to define dimension in algebraic topology using Zariskitopology is the following:
If X is an irreducible topological space, then any proper closed subsetof X must have dimension (at least) one smaller.
Surprisingly, it turns out that the right definition is purely topological:it depends only on the topological space.
Defintion (Dimension)
Let X be a topological space. Then the dimension dimX of X is definedto be the supremum of all integers n such that there exists a chain ofirreducible closed subsets of X :
X0 ( X1 ( · · · ( Xn
contained in X . In particular, if X is an affine variety, then we define thedimension of the variety to be its dimension as a topological space.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 3
/ 17
Dimension of an Affine Variety
Throughout the presentation, we assume the underlying field k to bealgebraically closed.
The idea to define dimension in algebraic topology using Zariskitopology is the following:
If X is an irreducible topological space, then any proper closed subsetof X must have dimension (at least) one smaller.
Surprisingly, it turns out that the right definition is purely topological:it depends only on the topological space.
Defintion (Dimension)
Let X be a topological space. Then the dimension dimX of X is definedto be the supremum of all integers n such that there exists a chain ofirreducible closed subsets of X :
X0 ( X1 ( · · · ( Xn
contained in X . In particular, if X is an affine variety, then we define thedimension of the variety to be its dimension as a topological space.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 3
/ 17
Dimension of an Affine Variety
Throughout the presentation, we assume the underlying field k to bealgebraically closed.
The idea to define dimension in algebraic topology using Zariskitopology is the following:
If X is an irreducible topological space, then any proper closed subsetof X must have dimension (at least) one smaller.
Surprisingly, it turns out that the right definition is purely topological:it depends only on the topological space.
Defintion (Dimension)
Let X be a topological space. Then the dimension dimX of X is definedto be the supremum of all integers n such that there exists a chain ofirreducible closed subsets of X :
X0 ( X1 ( · · · ( Xn
contained in X . In particular, if X is an affine variety, then we define thedimension of the variety to be its dimension as a topological space.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 3
/ 17
Dimension of an Affine Variety
Throughout the presentation, we assume the underlying field k to bealgebraically closed.
The idea to define dimension in algebraic topology using Zariskitopology is the following:
If X is an irreducible topological space, then any proper closed subsetof X must have dimension (at least) one smaller.
Surprisingly, it turns out that the right definition is purely topological:it depends only on the topological space.
Defintion (Dimension)
Let X be a topological space. Then the dimension dimX of X is definedto be the supremum of all integers n such that there exists a chain ofirreducible closed subsets of X :
X0 ( X1 ( · · · ( Xn
contained in X . In particular, if X is an affine variety, then we define thedimension of the variety to be its dimension as a topological space.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 3
/ 17
Dimension of an Affine Variety
Example
Recall from last time we proved that the only proper subvarieties of A1k are
the whole space and single points, hence dimA1k = 1.
This definition has the advantage of being short and intuitive, but ithas the disadvantage that it is very hard to apply in actualcomputations. To this end, we have an alternative and equivalentdefinition:
Theorem
Let X be an affine variety. Then dimX = tr .deg (k(X )/k), thetranscendence degree of k(X ) over k , where k(X ) is the field of rationalfunctions of X .
Proving the equivalence requires a lot of commutative algebra.
Example
The function field k(Ank) = k(x1, · · · , xn). Hence dimAn
k = n.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 4
/ 17
Dimension of an Affine Variety
Example
Recall from last time we proved that the only proper subvarieties of A1k are
the whole space and single points, hence dimA1k = 1.
This definition has the advantage of being short and intuitive, but ithas the disadvantage that it is very hard to apply in actualcomputations. To this end, we have an alternative and equivalentdefinition:
Theorem
Let X be an affine variety. Then dimX = tr .deg (k(X )/k), thetranscendence degree of k(X ) over k , where k(X ) is the field of rationalfunctions of X .
Proving the equivalence requires a lot of commutative algebra.
Example
The function field k(Ank) = k(x1, · · · , xn). Hence dimAn
k = n.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 4
/ 17
Dimension of an Affine Variety
Example
Recall from last time we proved that the only proper subvarieties of A1k are
the whole space and single points, hence dimA1k = 1.
This definition has the advantage of being short and intuitive, but ithas the disadvantage that it is very hard to apply in actualcomputations. To this end, we have an alternative and equivalentdefinition:
Theorem
Let X be an affine variety. Then dimX = tr .deg (k(X )/k), thetranscendence degree of k(X ) over k , where k(X ) is the field of rationalfunctions of X .
Proving the equivalence requires a lot of commutative algebra.
Example
The function field k(Ank) = k(x1, · · · , xn). Hence dimAn
k = n.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 4
/ 17
Dimension of an Affine Variety
Example
Recall from last time we proved that the only proper subvarieties of A1k are
the whole space and single points, hence dimA1k = 1.
This definition has the advantage of being short and intuitive, but ithas the disadvantage that it is very hard to apply in actualcomputations. To this end, we have an alternative and equivalentdefinition:
Theorem
Let X be an affine variety. Then dimX = tr .deg (k(X )/k), thetranscendence degree of k(X ) over k , where k(X ) is the field of rationalfunctions of X .
Proving the equivalence requires a lot of commutative algebra.
Example
The function field k(Ank) = k(x1, · · · , xn). Hence dimAn
k = n.Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine Varieties
Introduction to Algebraic Geometry, 2015 4/ 17
Maximal Ideal - Point Correspondence
Recall that we have a 1-1 correspondence between affine varieties Vand prime ideals in k[x1, · · · , xn]:{V ⊆ An
k} ←→ Spec k[x1, · · · , xn].
We also have:
Theorem (Maximal Ideal - Point Correspondence)
Let X be an affine variety with coordinate ring R = k[x1, · · · , xn]/I (X ).Then there is a bijective correspondence between the points p ∈ X andthe maximal ideals m E R.
Proof.
A maximal (hence prime) ideal m of k[x1, · · · , xn] corresponds to aminimal affine variety of An
k (because the correspondence is inclusionreversing), which must be a point p. The result follows from the 4thisomorphism theorem.
It implies that every maximal ideal of k[x1, · · · , xn] is of the formm = (x1 − a1, · · · , xn − an) for some ai ∈ k .
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 5
/ 17
Maximal Ideal - Point Correspondence
Recall that we have a 1-1 correspondence between affine varieties Vand prime ideals in k[x1, · · · , xn]:{V ⊆ An
k} ←→ Spec k[x1, · · · , xn]. We also have:
Theorem (Maximal Ideal - Point Correspondence)
Let X be an affine variety with coordinate ring R = k[x1, · · · , xn]/I (X ).Then there is a bijective correspondence between the points p ∈ X andthe maximal ideals m E R.
Proof.
A maximal (hence prime) ideal m of k[x1, · · · , xn] corresponds to aminimal affine variety of An
k (because the correspondence is inclusionreversing), which must be a point p. The result follows from the 4thisomorphism theorem.
It implies that every maximal ideal of k[x1, · · · , xn] is of the formm = (x1 − a1, · · · , xn − an) for some ai ∈ k .
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 5
/ 17
Maximal Ideal - Point Correspondence
Recall that we have a 1-1 correspondence between affine varieties Vand prime ideals in k[x1, · · · , xn]:{V ⊆ An
k} ←→ Spec k[x1, · · · , xn]. We also have:
Theorem (Maximal Ideal - Point Correspondence)
Let X be an affine variety with coordinate ring R = k[x1, · · · , xn]/I (X ).Then there is a bijective correspondence between the points p ∈ X andthe maximal ideals m E R.
Proof.
A maximal (hence prime) ideal m of k[x1, · · · , xn] corresponds to aminimal affine variety of An
k (because the correspondence is inclusionreversing), which must be a point p. The result follows from the 4thisomorphism theorem.
It implies that every maximal ideal of k[x1, · · · , xn] is of the formm = (x1 − a1, · · · , xn − an) for some ai ∈ k .
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 5
/ 17
Maximal Ideal - Point Correspondence
Recall that we have a 1-1 correspondence between affine varieties Vand prime ideals in k[x1, · · · , xn]:{V ⊆ An
k} ←→ Spec k[x1, · · · , xn]. We also have:
Theorem (Maximal Ideal - Point Correspondence)
Let X be an affine variety with coordinate ring R = k[x1, · · · , xn]/I (X ).Then there is a bijective correspondence between the points p ∈ X andthe maximal ideals m E R.
Proof.
A maximal (hence prime) ideal m of k[x1, · · · , xn] corresponds to aminimal affine variety of An
k (because the correspondence is inclusionreversing), which must be a point p. The result follows from the 4thisomorphism theorem.
It implies that every maximal ideal of k[x1, · · · , xn] is of the formm = (x1 − a1, · · · , xn − an) for some ai ∈ k .
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 5
/ 17
Tangent Space revisited
In calculus, we define the tangent plane to a surface by taking thespan of tengent vectors to arcs lying on it. We want to translate thisinto algebraic geometry.
Defintion (Derivations)
Let A→ B be a homomorphism of commutative algebras, and M aB-module. We define the derivations
DerA(B,M) = {D : B → M|D satisfying (1) and (2)}
(1) D(b1b2) = D(b1)b2 + b1D(b2) ∀b1, b2 ∈ B.(2) D(a) = 0 ∀a ∈ A.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 6
/ 17
Tangent Space revisited
In calculus, we define the tangent plane to a surface by taking thespan of tengent vectors to arcs lying on it. We want to translate thisinto algebraic geometry.
Defintion (Derivations)
Let A→ B be a homomorphism of commutative algebras, and M aB-module. We define the derivations
DerA(B,M) = {D : B → M|D satisfying (1) and (2)}
(1) D(b1b2) = D(b1)b2 + b1D(b2) ∀b1, b2 ∈ B.(2) D(a) = 0 ∀a ∈ A.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 6
/ 17
Motivation
Example
If A = k ,B = k[x1, · · · , xn], then M = k is a B-module. Define the map∂/∂xi : B → k by xj 7→ δij . We have
Derk(B, k) =
{D =
∑i
λi∂
∂xi: λi ∈ k
}.
Note that we have k ∼= OX ,p/mp. This is because the map k[X ]→ kgiven by evaluation at p has, by definition, kernel mp, and so does themap OX ,p
∼= k[X ]mp → k .
Defintion (Tangent Space)
Let X be an affine variety. The Zariski Tangent Space of X at p is definedas TpX = Derk(OX ,p, k).
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 7
/ 17
Motivation
Example
If A = k ,B = k[x1, · · · , xn], then M = k is a B-module. Define the map∂/∂xi : B → k by xj 7→ δij . We have
Derk(B, k) =
{D =
∑i
λi∂
∂xi: λi ∈ k
}.
Note that we have k ∼= OX ,p/mp. This is because the map k[X ]→ kgiven by evaluation at p has, by definition, kernel mp, and so does themap OX ,p
∼= k[X ]mp → k .
Defintion (Tangent Space)
Let X be an affine variety. The Zariski Tangent Space of X at p is definedas TpX = Derk(OX ,p, k).
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 7
/ 17
Motivation
Example
If A = k ,B = k[x1, · · · , xn], then M = k is a B-module. Define the map∂/∂xi : B → k by xj 7→ δij . We have
Derk(B, k) =
{D =
∑i
λi∂
∂xi: λi ∈ k
}.
Note that we have k ∼= OX ,p/mp. This is because the map k[X ]→ kgiven by evaluation at p has, by definition, kernel mp, and so does themap OX ,p
∼= k[X ]mp → k .
Defintion (Tangent Space)
Let X be an affine variety. The Zariski Tangent Space of X at p is definedas TpX = Derk(OX ,p, k).
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 7
/ 17
Tangent Spaces
Theorem
Let X be an affine variety and p ∈ X , then TpX ∼= (mp/m2p)∗, where V ∗ is
the dual vector space of V .
Proof.
Consider the map TpX → Hom (mp/m2p, k) defined by D → D = D|mp . D
vanishes on m2p because D(fg) = fD(g) + D(f )g ≡ 0 (mod mp) for
f , g ∈ mp. As a k-vector space, OX ,p∼= k ⊕mp, we see that D is
determined by mp. Hence D induces a linear map D : mp/m2p → k and
TpX → Hom (mp/m2p, k) is injective.On the other hand, let C = mp \m2
p
be the complement of m2p so that OX ,p
∼= k ⊕ C ⊕m2p. If T : C → k is
linear, then it is clear that the extension of T to a linear map D on OX ,p
by putting T |k⊕m2p
= 0 is a derviation in p.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 8
/ 17
Tangent Spaces
Theorem
Let X be an affine variety and p ∈ X , then TpX ∼= (mp/m2p)∗, where V ∗ is
the dual vector space of V .
Proof.
Consider the map TpX → Hom (mp/m2p, k) defined by D → D = D|mp .
Dvanishes on m2
p because D(fg) = fD(g) + D(f )g ≡ 0 (mod mp) forf , g ∈ mp. As a k-vector space, OX ,p
∼= k ⊕mp, we see that D isdetermined by mp. Hence D induces a linear map D : mp/m
2p → k and
TpX → Hom (mp/m2p, k) is injective.On the other hand, let C = mp \m2
p
be the complement of m2p so that OX ,p
∼= k ⊕ C ⊕m2p. If T : C → k is
linear, then it is clear that the extension of T to a linear map D on OX ,p
by putting T |k⊕m2p
= 0 is a derviation in p.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 8
/ 17
Tangent Spaces
Theorem
Let X be an affine variety and p ∈ X , then TpX ∼= (mp/m2p)∗, where V ∗ is
the dual vector space of V .
Proof.
Consider the map TpX → Hom (mp/m2p, k) defined by D → D = D|mp . D
vanishes on m2p because D(fg) = fD(g) + D(f )g ≡ 0 (mod mp) for
f , g ∈ mp. As a k-vector space, OX ,p∼= k ⊕mp, we see that D is
determined by mp.
Hence D induces a linear map D : mp/m2p → k and
TpX → Hom (mp/m2p, k) is injective.On the other hand, let C = mp \m2
p
be the complement of m2p so that OX ,p
∼= k ⊕ C ⊕m2p. If T : C → k is
linear, then it is clear that the extension of T to a linear map D on OX ,p
by putting T |k⊕m2p
= 0 is a derviation in p.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 8
/ 17
Tangent Spaces
Theorem
Let X be an affine variety and p ∈ X , then TpX ∼= (mp/m2p)∗, where V ∗ is
the dual vector space of V .
Proof.
Consider the map TpX → Hom (mp/m2p, k) defined by D → D = D|mp . D
vanishes on m2p because D(fg) = fD(g) + D(f )g ≡ 0 (mod mp) for
f , g ∈ mp. As a k-vector space, OX ,p∼= k ⊕mp, we see that D is
determined by mp. Hence D induces a linear map D : mp/m2p → k and
TpX → Hom (mp/m2p, k) is injective.
On the other hand, let C = mp \m2p
be the complement of m2p so that OX ,p
∼= k ⊕ C ⊕m2p. If T : C → k is
linear, then it is clear that the extension of T to a linear map D on OX ,p
by putting T |k⊕m2p
= 0 is a derviation in p.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 8
/ 17
Tangent Spaces
Theorem
Let X be an affine variety and p ∈ X , then TpX ∼= (mp/m2p)∗, where V ∗ is
the dual vector space of V .
Proof.
Consider the map TpX → Hom (mp/m2p, k) defined by D → D = D|mp . D
vanishes on m2p because D(fg) = fD(g) + D(f )g ≡ 0 (mod mp) for
f , g ∈ mp. As a k-vector space, OX ,p∼= k ⊕mp, we see that D is
determined by mp. Hence D induces a linear map D : mp/m2p → k and
TpX → Hom (mp/m2p, k) is injective.On the other hand, let C = mp \m2
p
be the complement of m2p so that OX ,p
∼= k ⊕ C ⊕m2p. If T : C → k is
linear, then it is clear that the extension of T to a linear map D on OX ,p
by putting T |k⊕m2p
= 0 is a derviation in p.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 8
/ 17
Smooth Varieties
Defintion (Hypersurfaces)
A hyersurface in Ank is a subvariety defined by a single equation
f (x1, · · · , xn) = 0.
Defintion (Singularities of a Hypersurface)
Let X be a hypersurface f (x1, · · · , xn) = 0 in Ank . A point p ∈ X is a
singularity of X if ∂f /∂xi (P) = 0 for all i = 1, · · · , n.
The set of singularities forms a subvariety of X , defined by f = 0together with the equations ∂f /∂xi = 0 for all i = 1, · · · , n.
Defintion (Smoothness)
A hypersurface X in Ank is called smooth (or nonsingular) if there are no
singularities in X .
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 9
/ 17
Smooth Varieties
Defintion (Hypersurfaces)
A hyersurface in Ank is a subvariety defined by a single equation
f (x1, · · · , xn) = 0.
Defintion (Singularities of a Hypersurface)
Let X be a hypersurface f (x1, · · · , xn) = 0 in Ank . A point p ∈ X is a
singularity of X if ∂f /∂xi (P) = 0 for all i = 1, · · · , n.
The set of singularities forms a subvariety of X , defined by f = 0together with the equations ∂f /∂xi = 0 for all i = 1, · · · , n.
Defintion (Smoothness)
A hypersurface X in Ank is called smooth (or nonsingular) if there are no
singularities in X .
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 9
/ 17
Smooth Varieties
Defintion (Hypersurfaces)
A hyersurface in Ank is a subvariety defined by a single equation
f (x1, · · · , xn) = 0.
Defintion (Singularities of a Hypersurface)
Let X be a hypersurface f (x1, · · · , xn) = 0 in Ank . A point p ∈ X is a
singularity of X if ∂f /∂xi (P) = 0 for all i = 1, · · · , n.
The set of singularities forms a subvariety of X , defined by f = 0together with the equations ∂f /∂xi = 0 for all i = 1, · · · , n.
Defintion (Smoothness)
A hypersurface X in Ank is called smooth (or nonsingular) if there are no
singularities in X .
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 9
/ 17
Smooth Varieties
Defintion (Hypersurfaces)
A hyersurface in Ank is a subvariety defined by a single equation
f (x1, · · · , xn) = 0.
Defintion (Singularities of a Hypersurface)
Let X be a hypersurface f (x1, · · · , xn) = 0 in Ank . A point p ∈ X is a
singularity of X if ∂f /∂xi (P) = 0 for all i = 1, · · · , n.
The set of singularities forms a subvariety of X , defined by f = 0together with the equations ∂f /∂xi = 0 for all i = 1, · · · , n.
Defintion (Smoothness)
A hypersurface X in Ank is called smooth (or nonsingular) if there are no
singularities in X .
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 9
/ 17
Motivating Example
Example (Elliptic Curve)
Let E be the elliptic curve y2 = x3 + 1 in A2Q, then the singular locus is
defined by the equations y2 − x3 − 1 = −3x2 = 2y = 0, which have nocommon solution in Q, so the curve is smooth.
Example (Nodes in a Singular Curve)
Let X be the ”nodal cubic” y2 = x3 + x2, then the singular locus isdefined by the equations y2 − x3 − x2 = −3x2 − 2x = 2y = 0,which have a unique solution (0, 0). Hence X is singular. It has two”branches” crossing at (0, 0). Such a singularity is called a node.
Example (Cusps in a Singular Curve)
Let Y be the curve y2 = x3, then Y is singular with a unique singularity(0, 0). It has no tangent at (0, 0). Such a singularity is called a cusp.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 10
/ 17
Motivating Example
Example (Elliptic Curve)
Let E be the elliptic curve y2 = x3 + 1 in A2Q, then the singular locus is
defined by the equations y2 − x3 − 1 = −3x2 = 2y = 0, which have nocommon solution in Q, so the curve is smooth.
Example (Nodes in a Singular Curve)
Let X be the ”nodal cubic” y2 = x3 + x2, then the singular locus isdefined by the equations y2 − x3 − x2 = −3x2 − 2x = 2y = 0,which have a unique solution (0, 0). Hence X is singular. It has two”branches” crossing at (0, 0). Such a singularity is called a node.
Example (Cusps in a Singular Curve)
Let Y be the curve y2 = x3, then Y is singular with a unique singularity(0, 0). It has no tangent at (0, 0). Such a singularity is called a cusp.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 10
/ 17
Motivating Example
Example (Elliptic Curve)
Let E be the elliptic curve y2 = x3 + 1 in A2Q, then the singular locus is
defined by the equations y2 − x3 − 1 = −3x2 = 2y = 0, which have nocommon solution in Q, so the curve is smooth.
Example (Nodes in a Singular Curve)
Let X be the ”nodal cubic” y2 = x3 + x2, then the singular locus isdefined by the equations y2 − x3 − x2 = −3x2 − 2x = 2y = 0,which have a unique solution (0, 0). Hence X is singular. It has two”branches” crossing at (0, 0). Such a singularity is called a node.
Example (Cusps in a Singular Curve)
Let Y be the curve y2 = x3, then Y is singular with a unique singularity(0, 0). It has no tangent at (0, 0). Such a singularity is called a cusp.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 10
/ 17
Smooth Varieties
More generally:
Defintion (Smoothness)
An affine variety X = V (f1, · · · , fm) is smooth (or nonsingular) at p if them × n Jacobian matrix has rank n − dimX , i.e.
rk
(∂fi∂xj
(p)
)1≤i≤m1≤j≤n
= n − dimX .
X is nonsingular (or smooth) if X is nonsingular at every point.
When X is generated by a single non-constant polynomial, then thedefiniton reduces to the case of hypersurface.
If X = V (f ) is a hypersurface, then p ∈ X is a singularity iff∂f /∂xi (p) = 0 for all i = 1, · · · , n and f (P) = 0, this gives n + 1equations in n variables. Thus for a randomly chosen hypersurface X ,we would expect X to be nonsingular.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 11
/ 17
Smooth Varieties
More generally:
Defintion (Smoothness)
An affine variety X = V (f1, · · · , fm) is smooth (or nonsingular) at p if them × n Jacobian matrix has rank n − dimX , i.e.
rk
(∂fi∂xj
(p)
)1≤i≤m1≤j≤n
= n − dimX .
X is nonsingular (or smooth) if X is nonsingular at every point.
When X is generated by a single non-constant polynomial, then thedefiniton reduces to the case of hypersurface.
If X = V (f ) is a hypersurface, then p ∈ X is a singularity iff∂f /∂xi (p) = 0 for all i = 1, · · · , n and f (P) = 0, this gives n + 1equations in n variables. Thus for a randomly chosen hypersurface X ,we would expect X to be nonsingular.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 11
/ 17
Smooth Varieties
More generally:
Defintion (Smoothness)
An affine variety X = V (f1, · · · , fm) is smooth (or nonsingular) at p if them × n Jacobian matrix has rank n − dimX , i.e.
rk
(∂fi∂xj
(p)
)1≤i≤m1≤j≤n
= n − dimX .
X is nonsingular (or smooth) if X is nonsingular at every point.
When X is generated by a single non-constant polynomial, then thedefiniton reduces to the case of hypersurface.
If X = V (f ) is a hypersurface, then p ∈ X is a singularity iff∂f /∂xi (p) = 0 for all i = 1, · · · , n and f (P) = 0, this gives n + 1equations in n variables. Thus for a randomly chosen hypersurface X ,we would expect X to be nonsingular.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 11
/ 17
Equivalent Definitions of Smoothness
Our definiton of smoothness has a drawback in the sense that itdepends on the embedding of X in affine space.
Zariski proved that nonsingularity is intrinsic:
Theorem (Zariski, 1947)
Let X ⊆ Ank be an affine variety and p ∈ X . Then X is nonsingular at P iff
dimk[X ]/mp(mp/m
2p) = dimX .
This theorem is important because it implies that nonsingularity isintrinsic, i.e. it could be described in terms of the local rings, ratherthan a set of generators.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 12
/ 17
Equivalent Definitions of Smoothness
Our definiton of smoothness has a drawback in the sense that itdepends on the embedding of X in affine space.
Zariski proved that nonsingularity is intrinsic:
Theorem (Zariski, 1947)
Let X ⊆ Ank be an affine variety and p ∈ X . Then X is nonsingular at P iff
dimk[X ]/mp(mp/m
2p) = dimX .
This theorem is important because it implies that nonsingularity isintrinsic, i.e. it could be described in terms of the local rings, ratherthan a set of generators.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 12
/ 17
Equivalent Definitions of Smoothness
Our definiton of smoothness has a drawback in the sense that itdepends on the embedding of X in affine space.
Zariski proved that nonsingularity is intrinsic:
Theorem (Zariski, 1947)
Let X ⊆ Ank be an affine variety and p ∈ X . Then X is nonsingular at P iff
dimk[X ]/mp(mp/m
2p) = dimX .
This theorem is important because it implies that nonsingularity isintrinsic, i.e. it could be described in terms of the local rings, ratherthan a set of generators.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 12
/ 17
Equivalent Definitions of Smoothness
Our definiton of smoothness has a drawback in the sense that itdepends on the embedding of X in affine space.
Zariski proved that nonsingularity is intrinsic:
Theorem (Zariski, 1947)
Let X ⊆ Ank be an affine variety and p ∈ X . Then X is nonsingular at P iff
dimk[X ]/mp(mp/m
2p) = dimX .
This theorem is important because it implies that nonsingularity isintrinsic, i.e. it could be described in terms of the local rings, ratherthan a set of generators.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 12
/ 17
Equivalent Definitions of Smoothness
Proof.
First we can identify k[X ]/mp with k (why?).
Let p = (a1, · · · , an) ∈ Ank
and let ap = (x1 − a1, · · · , xn − an). We define a map byϕ : k[x1, · · · , xn]→ kn by ϕ(f ) = (∂f /∂x1(p), · · · , ∂f /∂xn(p)). Then it isclear that ϕ(xi − ai ), i = 1, · · · , n form a basis of kn and ϕ(a2
p) = 0.Hence ϕ induces an isomorphism ϕ : ap/a
2p → kn. Now write X = V (b)
and let f1, · · · , ft be a set of generators of b E k[x1, · · · , xn] (this can bedone because k[x1, · · · , xn] is Noetherian). Thenrk(∂fi/∂xj(p)) = dimϕ(b).Using the isomorphism ϕ, this is the same asdim b/(b ∩ a2
p) = dim(b + a2p)/a2
p. Now, OX ,p∼= k[X ]ap . Hence if m is the
maximal ideal of OX ,p, we have m/m2 ∼= ap/(b + a2p). Counting
dimensions of vector spaces, we have dimm/m2 + rk(∂fi/∂xj(p)) = n. Theresult follows immediately.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 13
/ 17
Equivalent Definitions of Smoothness
Proof.
First we can identify k[X ]/mp with k (why?). Let p = (a1, · · · , an) ∈ Ank
and let ap = (x1 − a1, · · · , xn − an). We define a map byϕ : k[x1, · · · , xn]→ kn by ϕ(f ) = (∂f /∂x1(p), · · · , ∂f /∂xn(p)).
Then it isclear that ϕ(xi − ai ), i = 1, · · · , n form a basis of kn and ϕ(a2
p) = 0.Hence ϕ induces an isomorphism ϕ : ap/a
2p → kn. Now write X = V (b)
and let f1, · · · , ft be a set of generators of b E k[x1, · · · , xn] (this can bedone because k[x1, · · · , xn] is Noetherian). Thenrk(∂fi/∂xj(p)) = dimϕ(b).Using the isomorphism ϕ, this is the same asdim b/(b ∩ a2
p) = dim(b + a2p)/a2
p. Now, OX ,p∼= k[X ]ap . Hence if m is the
maximal ideal of OX ,p, we have m/m2 ∼= ap/(b + a2p). Counting
dimensions of vector spaces, we have dimm/m2 + rk(∂fi/∂xj(p)) = n. Theresult follows immediately.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 13
/ 17
Equivalent Definitions of Smoothness
Proof.
First we can identify k[X ]/mp with k (why?). Let p = (a1, · · · , an) ∈ Ank
and let ap = (x1 − a1, · · · , xn − an). We define a map byϕ : k[x1, · · · , xn]→ kn by ϕ(f ) = (∂f /∂x1(p), · · · , ∂f /∂xn(p)). Then it isclear that ϕ(xi − ai ), i = 1, · · · , n form a basis of kn and ϕ(a2
p) = 0.Hence ϕ induces an isomorphism ϕ : ap/a
2p → kn.
Now write X = V (b)and let f1, · · · , ft be a set of generators of b E k[x1, · · · , xn] (this can bedone because k[x1, · · · , xn] is Noetherian). Thenrk(∂fi/∂xj(p)) = dimϕ(b).Using the isomorphism ϕ, this is the same asdim b/(b ∩ a2
p) = dim(b + a2p)/a2
p. Now, OX ,p∼= k[X ]ap . Hence if m is the
maximal ideal of OX ,p, we have m/m2 ∼= ap/(b + a2p). Counting
dimensions of vector spaces, we have dimm/m2 + rk(∂fi/∂xj(p)) = n. Theresult follows immediately.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 13
/ 17
Equivalent Definitions of Smoothness
Proof.
First we can identify k[X ]/mp with k (why?). Let p = (a1, · · · , an) ∈ Ank
and let ap = (x1 − a1, · · · , xn − an). We define a map byϕ : k[x1, · · · , xn]→ kn by ϕ(f ) = (∂f /∂x1(p), · · · , ∂f /∂xn(p)). Then it isclear that ϕ(xi − ai ), i = 1, · · · , n form a basis of kn and ϕ(a2
p) = 0.Hence ϕ induces an isomorphism ϕ : ap/a
2p → kn. Now write X = V (b)
and let f1, · · · , ft be a set of generators of b E k[x1, · · · , xn] (this can bedone because k[x1, · · · , xn] is Noetherian).
Thenrk(∂fi/∂xj(p)) = dimϕ(b).Using the isomorphism ϕ, this is the same asdim b/(b ∩ a2
p) = dim(b + a2p)/a2
p. Now, OX ,p∼= k[X ]ap . Hence if m is the
maximal ideal of OX ,p, we have m/m2 ∼= ap/(b + a2p). Counting
dimensions of vector spaces, we have dimm/m2 + rk(∂fi/∂xj(p)) = n. Theresult follows immediately.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 13
/ 17
Equivalent Definitions of Smoothness
Proof.
First we can identify k[X ]/mp with k (why?). Let p = (a1, · · · , an) ∈ Ank
and let ap = (x1 − a1, · · · , xn − an). We define a map byϕ : k[x1, · · · , xn]→ kn by ϕ(f ) = (∂f /∂x1(p), · · · , ∂f /∂xn(p)). Then it isclear that ϕ(xi − ai ), i = 1, · · · , n form a basis of kn and ϕ(a2
p) = 0.Hence ϕ induces an isomorphism ϕ : ap/a
2p → kn. Now write X = V (b)
and let f1, · · · , ft be a set of generators of b E k[x1, · · · , xn] (this can bedone because k[x1, · · · , xn] is Noetherian). Thenrk(∂fi/∂xj(p)) = dimϕ(b).
Using the isomorphism ϕ, this is the same asdim b/(b ∩ a2
p) = dim(b + a2p)/a2
p. Now, OX ,p∼= k[X ]ap . Hence if m is the
maximal ideal of OX ,p, we have m/m2 ∼= ap/(b + a2p). Counting
dimensions of vector spaces, we have dimm/m2 + rk(∂fi/∂xj(p)) = n. Theresult follows immediately.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 13
/ 17
Equivalent Definitions of Smoothness
Proof.
First we can identify k[X ]/mp with k (why?). Let p = (a1, · · · , an) ∈ Ank
and let ap = (x1 − a1, · · · , xn − an). We define a map byϕ : k[x1, · · · , xn]→ kn by ϕ(f ) = (∂f /∂x1(p), · · · , ∂f /∂xn(p)). Then it isclear that ϕ(xi − ai ), i = 1, · · · , n form a basis of kn and ϕ(a2
p) = 0.Hence ϕ induces an isomorphism ϕ : ap/a
2p → kn. Now write X = V (b)
and let f1, · · · , ft be a set of generators of b E k[x1, · · · , xn] (this can bedone because k[x1, · · · , xn] is Noetherian). Thenrk(∂fi/∂xj(p)) = dimϕ(b).Using the isomorphism ϕ, this is the same asdim b/(b ∩ a2
p) = dim(b + a2p)/a2
p.
Now, OX ,p∼= k[X ]ap . Hence if m is the
maximal ideal of OX ,p, we have m/m2 ∼= ap/(b + a2p). Counting
dimensions of vector spaces, we have dimm/m2 + rk(∂fi/∂xj(p)) = n. Theresult follows immediately.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 13
/ 17
Equivalent Definitions of Smoothness
Proof.
First we can identify k[X ]/mp with k (why?). Let p = (a1, · · · , an) ∈ Ank
and let ap = (x1 − a1, · · · , xn − an). We define a map byϕ : k[x1, · · · , xn]→ kn by ϕ(f ) = (∂f /∂x1(p), · · · , ∂f /∂xn(p)). Then it isclear that ϕ(xi − ai ), i = 1, · · · , n form a basis of kn and ϕ(a2
p) = 0.Hence ϕ induces an isomorphism ϕ : ap/a
2p → kn. Now write X = V (b)
and let f1, · · · , ft be a set of generators of b E k[x1, · · · , xn] (this can bedone because k[x1, · · · , xn] is Noetherian). Thenrk(∂fi/∂xj(p)) = dimϕ(b).Using the isomorphism ϕ, this is the same asdim b/(b ∩ a2
p) = dim(b + a2p)/a2
p. Now, OX ,p∼= k[X ]ap .
Hence if m is themaximal ideal of OX ,p, we have m/m2 ∼= ap/(b + a2
p). Countingdimensions of vector spaces, we have dimm/m2 + rk(∂fi/∂xj(p)) = n. Theresult follows immediately.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 13
/ 17
Equivalent Definitions of Smoothness
Proof.
First we can identify k[X ]/mp with k (why?). Let p = (a1, · · · , an) ∈ Ank
and let ap = (x1 − a1, · · · , xn − an). We define a map byϕ : k[x1, · · · , xn]→ kn by ϕ(f ) = (∂f /∂x1(p), · · · , ∂f /∂xn(p)). Then it isclear that ϕ(xi − ai ), i = 1, · · · , n form a basis of kn and ϕ(a2
p) = 0.Hence ϕ induces an isomorphism ϕ : ap/a
2p → kn. Now write X = V (b)
and let f1, · · · , ft be a set of generators of b E k[x1, · · · , xn] (this can bedone because k[x1, · · · , xn] is Noetherian). Thenrk(∂fi/∂xj(p)) = dimϕ(b).Using the isomorphism ϕ, this is the same asdim b/(b ∩ a2
p) = dim(b + a2p)/a2
p. Now, OX ,p∼= k[X ]ap . Hence if m is the
maximal ideal of OX ,p, we have m/m2 ∼= ap/(b + a2p).
Countingdimensions of vector spaces, we have dimm/m2 + rk(∂fi/∂xj(p)) = n. Theresult follows immediately.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 13
/ 17
Equivalent Definitions of Smoothness
Proof.
First we can identify k[X ]/mp with k (why?). Let p = (a1, · · · , an) ∈ Ank
and let ap = (x1 − a1, · · · , xn − an). We define a map byϕ : k[x1, · · · , xn]→ kn by ϕ(f ) = (∂f /∂x1(p), · · · , ∂f /∂xn(p)). Then it isclear that ϕ(xi − ai ), i = 1, · · · , n form a basis of kn and ϕ(a2
p) = 0.Hence ϕ induces an isomorphism ϕ : ap/a
2p → kn. Now write X = V (b)
and let f1, · · · , ft be a set of generators of b E k[x1, · · · , xn] (this can bedone because k[x1, · · · , xn] is Noetherian). Thenrk(∂fi/∂xj(p)) = dimϕ(b).Using the isomorphism ϕ, this is the same asdim b/(b ∩ a2
p) = dim(b + a2p)/a2
p. Now, OX ,p∼= k[X ]ap . Hence if m is the
maximal ideal of OX ,p, we have m/m2 ∼= ap/(b + a2p). Counting
dimensions of vector spaces, we have dimm/m2 + rk(∂fi/∂xj(p)) = n.
Theresult follows immediately.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 13
/ 17
Equivalent Definitions of Smoothness
Proof.
First we can identify k[X ]/mp with k (why?). Let p = (a1, · · · , an) ∈ Ank
and let ap = (x1 − a1, · · · , xn − an). We define a map byϕ : k[x1, · · · , xn]→ kn by ϕ(f ) = (∂f /∂x1(p), · · · , ∂f /∂xn(p)). Then it isclear that ϕ(xi − ai ), i = 1, · · · , n form a basis of kn and ϕ(a2
p) = 0.Hence ϕ induces an isomorphism ϕ : ap/a
2p → kn. Now write X = V (b)
and let f1, · · · , ft be a set of generators of b E k[x1, · · · , xn] (this can bedone because k[x1, · · · , xn] is Noetherian). Thenrk(∂fi/∂xj(p)) = dimϕ(b).Using the isomorphism ϕ, this is the same asdim b/(b ∩ a2
p) = dim(b + a2p)/a2
p. Now, OX ,p∼= k[X ]ap . Hence if m is the
maximal ideal of OX ,p, we have m/m2 ∼= ap/(b + a2p). Counting
dimensions of vector spaces, we have dimm/m2 + rk(∂fi/∂xj(p)) = n. Theresult follows immediately.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 13
/ 17
Equivalent Definitions of Smoothness
In fact, an even stronger result is true:
Theorem
Let X ⊆ Ank be an affine variety with corrdinate ring k[X ] ∼= O(X ), then
X is nonsingular at p iff the localization k[X ]mp∼= OX ,p of k[X ] at mp is a
UFD.
Proving the equivalence requires a lot of commutative algebra.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 14
/ 17
Equivalent Definitions of Smoothness
In fact, an even stronger result is true:
Theorem
Let X ⊆ Ank be an affine variety with corrdinate ring k[X ] ∼= O(X ), then
X is nonsingular at p iff the localization k[X ]mp∼= OX ,p of k[X ] at mp is a
UFD.
Proving the equivalence requires a lot of commutative algebra.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 14
/ 17
Equivalent Definitions of Smoothness
To summarize:
Theorem
Let X = V (f1, · · · , fm) ⊆ Ank be an affine variety and let p ∈ X . Then the
following are equivalent:(1) X is smooth (or nonsingular) at p,(2) the m × n Jacobian matrix has rank n − dimX , i.e.
rk
(∂fi∂xj
(p)
)1≤i≤m1≤j≤n
= n − dimX ,
(3) dimk[X ]/mp(mp/m
2p) = dimX ,
(4) The localization k[X ]mp∼= OX ,p of k[X ] at mp is a UFD.
Food for thought: Verify that the localization of k[x , y ]/(y2 − x3)and k[x , y ]/(y2 − x3 − x2) at p = (0, 0) is not a UFD.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 15
/ 17
Equivalent Definitions of Smoothness
To summarize:
Theorem
Let X = V (f1, · · · , fm) ⊆ Ank be an affine variety and let p ∈ X . Then the
following are equivalent:(1) X is smooth (or nonsingular) at p,(2) the m × n Jacobian matrix has rank n − dimX , i.e.
rk
(∂fi∂xj
(p)
)1≤i≤m1≤j≤n
= n − dimX ,
(3) dimk[X ]/mp(mp/m
2p) = dimX ,
(4) The localization k[X ]mp∼= OX ,p of k[X ] at mp is a UFD.
Food for thought: Verify that the localization of k[x , y ]/(y2 − x3)and k[x , y ]/(y2 − x3 − x2) at p = (0, 0) is not a UFD.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 15
/ 17
Equivalent Definitions of Smoothness
To summarize:
Theorem
Let X = V (f1, · · · , fm) ⊆ Ank be an affine variety and let p ∈ X . Then the
following are equivalent:(1) X is smooth (or nonsingular) at p,(2) the m × n Jacobian matrix has rank n − dimX , i.e.
rk
(∂fi∂xj
(p)
)1≤i≤m1≤j≤n
= n − dimX ,
(3) dimk[X ]/mp(mp/m
2p) = dimX ,
(4) The localization k[X ]mp∼= OX ,p of k[X ] at mp is a UFD.
Food for thought: Verify that the localization of k[x , y ]/(y2 − x3)and k[x , y ]/(y2 − x3 − x2) at p = (0, 0) is not a UFD.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 15
/ 17
References
[1] M. Atiyah, Introduction to Commutative Algebra, Westview Press,1994.
[2] D. Eisenbud, Commutative Algebra: with a View Toward AlgebraicGeometry. Springer-Verlag, 1990.
[3] I. Shafarevich - Basic Algebraic Geometry 1, Springer-Verlag, 2007.
[4] R. Hartshorne, Algebraic Geometry. Springer-Verlag, 1977.
[5] K. Ueno - Algebraic Geometry 1: From Algebraic Varieties to Schemes,American Mathematical Society, 1999.
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 16
/ 17
Thanks
Happy Easter everyone!
Ryan, Lok-Wing Pang (HKUST) Dimensions, Tangent Spaces and Smooth Affine VarietiesIntroduction to Algebraic Geometry, 2015 17
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