In this section we will give a quick review on rational homogeneous va-rieties and their description in terms of quotients of algebraic groups.

**Definition 2.2.1.** *(Bor91, 11.2) Let* 𝐺 *be a connected affine group. A*
parabolic subgroup 𝑃 ⊂ 𝐺 *is a closed subgroup such that the quotient*
𝐺/𝑃*is a complete variety.*

One observes that a homogeneous space of the form 𝐺/𝑃 is always quasi-projective (Bor91, 6.8), then 𝐺/𝑃 is a projective variety if and only if 𝑃 is parabolic. Furthermore, for 𝐺 linear, reduced and con-nected, 𝐺/𝑃 is Fano for every 𝑃 (Kol96, Chapter V, Theorem 1.4).

Algebraic groups of the form 𝐺/𝑃, where 𝐺 is simple and 𝑃 is parabolic, are particularly interesting due to the following result, due to Borel and Remmert:

**Theorem 2.2.2.** *A rational homogeneous variety* 𝑋 *is isomorphic to a*
*product*

𝑋 =𝐺_{1}/𝑃_{1}× · · · ×𝐺_{𝑛}/𝑃_{𝑛} (2.2.1)
*where*𝐺_{𝑖}*is a simple group and*𝑃_{𝑖}*is a parabolic subgroup for*1≤ 𝑖 ≤ 𝑛*.*

**2.2.1** **Basic Lie algebra theory**

Let us first recall some basic information on Lie algebras, which can be found, for example, in (Cor97), and fix the notation for the next paragraphs.

**Cartan subalgebras**

**Definition 2.2.3.** *(Hel62, Ch. III, Sec. 3) Let*𝔤*be a complex semisimple*
*Lie algebra. A*Cartan subalgebra 𝔥 ⊂ 𝔤*is a subalgebra satisfying the*
*following conditions:*

◦ *it is a maximal abelian subalgebra of*𝔤

◦ *the adjoint representation ad*(ℎ)

*is completely reducible for every*

*element*ℎ ∈𝔥.

Such subalgebra always exists (Hel62, Theorem 4.1) and it is unique up to automorphism of 𝔤 (Hel62, Ch. III, Sec. 5)

Let us consider a basis{ℎ_{1},· · · , ℎ_{𝑟}} of the Cartan subalgebra𝔥of an𝑛
-dimensional semisimple Lie algebra𝔤of rank𝑟. Since𝔥is abelian, then
the matrices {ad(ℎ_{1}), . . . ,**ad(**ℎ_{𝑟})} are simultaneously diagonalizable.

Hence, there exists a basis {ℎ_{1},· · · , ℎ_{𝑟}, 𝑎_{𝑟}_{+1},· · ·𝑎_{𝑛}} of 𝔤 such that
the action of **ad(**ℎ_{𝑖}) acts on the 𝑎_{𝑗}’s as multiplication by constant.

Thus, since by definition **ad(**ℎ_{𝑖}) (−) = [ℎ_{𝑖},−], we can introduce the
notation

[ℎ_{𝑖}, 𝑎_{𝑗}] =𝛼_{𝑗}(ℎ_{𝑖})𝑎_{𝑗}. (2.2.2)
By linearity, Equation 2.2.2 defines for every basis element 𝑎_{𝑖} a linear
functional called a *root* of 𝔤 with respect to 𝔥 (Cor97, Chapter 13,
Section 4):

𝛼_{𝑖} :𝔥 C

ℎ 𝛼_{𝑖}(ℎ).

(2.2.3)

Let us now consider the *Killing form*

𝐾(𝑥 , 𝑦) :=tr(ad(𝑥)ad(𝑦)). (2.2.4)
Once restricted to 𝔥 it provides a nondegenerate symmetric bilinear
form (Cor97, Chapter 13, Theorem III), which can be used to associate
to each root 𝛼 a unique element ℎ_{𝛼} ∈𝔥 by:

𝐾(ℎ_{𝛼}, ℎ) =𝛼(ℎ). (2.2.5)

We also introduce the notation h𝛼_{1}, 𝛼_{2}i:=𝐾(ℎ_{𝛼}

1, ℎ_{𝛼}

2).
**Theorem 2.2.4.** *(Cor97, Chapter 13, Section 5, Theorem V)*
*Let* {ℎ_{𝛼}

1, . . . , ℎ_{𝛼}

𝑟} ⊂ 𝔥 *be a linearly independent set, for some roots*
{𝛼_{1}, . . . , 𝛼_{𝑟}}*. Then every root*𝛼*can be written as*𝛼= 𝑘_{1}𝛼_{1}+ · · · +𝑘_{𝑟}𝛼_{𝑟}
*with real and rational coefficients*𝑘_{1}, . . . , 𝑘_{𝑟}*.*

**Definition 2.2.5.** *(Cor97, Chapter 13, Section 7) Let*{ℎ_{𝛼}

1, . . . , ℎ_{𝛼}

𝑟} ⊂𝔥
*be a linearly independent set, for some roots* {𝛼_{1}, . . . , 𝛼_{𝑟}}. A non zero
*root* 𝛼 *is said to be* positive *with respect to* {𝛼_{1}, . . . , 𝛼_{𝑟}} *if the first*
*nonvanishing coefficient* 𝑘_{𝑖} *of the expression* 𝛼 = 𝑘_{1}𝛼_{1}+ · · · +𝑘_{𝑟}𝛼_{𝑟} *is*
*positive.*

**Definition 2.2.6.** *(Cor97, Chapter 13, Section 7) A non zero root* 𝛼 *is*
*said to be* simple*with respect to* {𝛼_{1}, . . . , 𝛼_{𝑟}} *if it cannot be expressed*
*as* 𝛼 = 𝛼^{0} + 𝛼^{00}*, where* 𝛼^{0} *and* 𝛼^{00} *are positive roots with respect to*
{𝛼_{1}, . . . , 𝛼_{𝑟}}*.*

**Theorem 2.2.7.** *(Cor97, Chapter 13, Section 7, Theorem II)*

*Let* {𝛼_{1}, . . . , 𝛼_{𝑟}} *be simple roots. Then every positive root* 𝛼 *can be*
*written as*𝛼=𝑘_{1}𝛼_{1}+ · · · +𝑘_{𝑟}𝛼_{𝑟} *for nonnegative integers*𝑘_{1}, . . . , 𝑘_{𝑟}*.*
Hereafter we will denote Δ the set of roots, andΔ± the subset of
posi-tive (negaposi-tive) roots, while the set of simple roots will be called S.

**The classification of semisimple Lie algebras**

Let us fix a Lie algebra 𝔤 of rank 𝑟 with a Cartan subalgebra 𝔥 and
a set of simple roots S = {𝛼_{1},· · · , 𝛼_{𝑟}}. Then, we can define a 𝑟 ×𝑟

invertible matrix called *Cartan matrix* in the following way:

𝐴_{𝑖 𝑗} := 2h𝛼_{𝑖}, 𝛼_{𝑗}i

h𝛼_{𝑖}, 𝛼_{𝑖}i (2.2.6)

Every root of 𝔤 can be recovered by the Cartan matrix and a choice of
S by *Weyl reflections:*

𝑆_{𝛼}

𝑖 :𝛼_{𝑗} ↦−→𝛼_{𝑗} − 𝐴_{𝑖 𝑗}𝛼_{𝑖} (2.2.7)
Note that all the information required to characterize a semisimple Lie
algebra is encoded in its Cartan matrix. In fact, one can prove that the
entries of such matrix can only be integers smaller or equal than three,
and that there exist only a finite set of admissible Cartan matrices. This
fact leads to the famous Dynkin-type classification of semisimple Lie
algebras:

𝐴_{𝑛} 𝑛 ≥ 1

𝐵_{𝑛} 𝑛 ≥ 2

𝐶_{𝑛} 𝑛 ≥ 2

𝐷_{𝑛} 𝑛 ≥ 4

𝐸_{𝑛} 6 ≤ 𝑛≤ 8

𝐹_{4}
𝐺_{2}

(2.2.8)

**Weights and representations**

Hereafter we will review how representations of semisimple Lie alge-bras can be characterized by their highest weight. This will fix

back-ground and notation in order to introduce homogeneous vector bundles.

Let us fix a complex semisimple Lie algebra 𝔤 of dimension 𝑛 and
rank 𝑟, and a Cartan subalgebra 𝔥 ⊂ 𝔤. Furthermore, let us choose
a set of simple roots S = {𝛼_{1},· · · , 𝛼_{𝑟}}. Let Γ be a 𝑁-dimensional
representation of 𝔤, i.e. a homomorphism of the following kind:

Γ:𝔤−→End𝑉_{Γ} (2.2.9)

where𝑉_{Γ}'C^{𝑁}. The matricesΓ(ℎ) for every ℎ ∈𝔥can be diagonalized
simultaneously (Cor97, Chapter 15, Section 2) hence let us assume
they are diagonal. Then, the *weights* of Γ are the following linear
functionals:

Λ𝑗 :𝔥 C ℎ Γ𝑗 𝑗(ℎ)

(2.2.10)

In other words, evaluations of weights on an element ℎ are eigenvalues
of the diagonal operatorsΓ(ℎ) ∈End𝑉_{Γ} (Cor97, Ch. 15, Sec. 2).

**Theorem 2.2.8.** *(Cor97, Chapter 15, Theorem III) Every weight* 𝜔 *can*
*be written as*𝜔 = 𝑘_{1}𝛼_{1}+ · · · +𝑘_{𝑟}𝛼_{𝑟}*, where* 𝛼_{1}, . . . , 𝛼_{𝑟} *are simple roots*
*and all coefficients* 𝑘_{1}, . . . , 𝑘_{𝑟} *are real and rational.*

Weyl reflections can be extended to weights: in other words, one can
define a Weyl reflection 𝑆_{𝛼}(𝜔) for a weight 𝜔 and a simple root 𝛼
by expressing 𝜔 as linear combinations of simple roots. Moreover,
one calls *positive* a weight 𝜔 = 𝑘_{1}𝛼_{1} + · · · +𝑘_{𝑟}𝛼_{𝑟} such that the first
nonvanishing 𝑘_{𝑖} is positive, and introduce a partial ordering among
weights: namely, we say 𝜔 > 𝜔^{0} if𝜔−𝜔^{0} is a positive weight. We call

*highest weight* of a representation a unique weight Λ such that Λ > 𝜔
for every other weight 𝜔.

One defines a distinguished set of *fundamental weights*in the following
way (Cor97, Chapter 15, Section 3):

𝜔_{𝑗} =

𝑟

Õ

𝑘=1

(𝐴^{−1})𝑘 𝑗𝛼_{𝑘}(ℎ) (2.2.11)
The highest weight of an irreducible representation can be written as
a linear combination of the fundamental weights with nonnegative
in-tegeral coefficients (Cor97, Chapter 15, Section 3, Theorem I). In the
following, given a representation Γ𝜔 of highest weight𝜔 =Í

𝑖𝜆_{𝑖}𝜔_{𝑖}, we
will denote such weight as

𝜔 = (𝜆_{1}, . . . , 𝜆_{𝑟}).

**Definition 2.2.9.** *A weight*𝜆 = Í

𝑖𝜆_{𝑖}𝜔_{𝑖} *is called* dominant*if*𝜆_{𝑖} ≥ 0*for*
*every*𝑖

**Definition 2.2.10.** *We call* length *of a Weyl reflection* 𝑆 *the minimal*
*integer* 𝑙(𝑆) *such that* 𝑆 *is a composition of* 𝑙(𝑆) *Weyl reflections with*
*respect to s imple roots.*

**2.2.2** **Levi decomposition and parabolic subgroups**

There exists a nice combinatoric description of parabolic subgroups of a linear reductive algebraic group𝐺, which will be reviewed here. We follow (IMOU16) and the sources therein.

Let us fix a linear reductive algebraic group 𝐺, and the corresponding Lie algebra 𝔤. Then 𝔤 decomposes in the following direct sum:

𝔤 = 𝔥⊕Ê

𝛼∈Δ

𝔤𝛼 (2.2.12)

where the root spaces 𝔤𝛼 are given by

𝔤𝛼 ={𝑔 ∈𝔤: [ℎ, 𝑔] =𝛼(ℎ)𝑔 for ℎ ∈𝔥} (2.2.13)
Inside 𝔤, one distinguishes the *standard Borel subalgebra* given by the
expression

𝔟 = 𝔥⊕𝔫 (2.2.14) where 𝔫 = É

𝛼∈Δ+𝔤𝛼. A useful characterization of parabolic
subalge-bras is given in terms of subalgesubalge-bras containing 𝔟. The direct sum
de-composition of Equation 2.2.12 allows us to wrote such subalgebras in
a particularly convenient way. Given the chosen set S = {𝛼_{1}, . . . , 𝛼_{𝑟}}
one can define nilpotent subalgebras of the form

𝔲𝑖_{1}...𝑖𝑙 = Ê

is a subalgebra and contains 𝔟, hence it is parabolic. The list of
parabolic subalgebras {𝔭𝑖_{1},...,𝑖_{𝑙}} is exhaustive up to conjugation.

*Remark* 2.2.11. Given a semisimple Lie group 𝐺 and parabolic
sub-group 𝑃 such that 𝐵 ⊂ 𝑃, the maps

𝜋 :𝐺/𝐵 −→𝐺/𝑃 (2.2.17)

are fiber bundles with fiber isomorphic to𝑃/𝐵, which is a homogeneous
variety as well (see for example the notes (Ott95, Page 55) and the
references therein). In particular, if we choose 𝑃 such that 𝜋 is an
extremal contraction, it follows that 𝜋 is a P^{1}-bundle. This tells us that
a complete 𝐺-flag variety has 𝑟 distinct P^{1}-bundle structures, where 𝑟
is the rank of 𝐺.

**2.2.3** **Example: Grassmannians and flags**

As an example, let us work out the case of 𝔤 =sl(𝑛,C), the algebra of
traceless𝑛×𝑛 matrices, corresponding to the group𝐺 =𝑆 𝐿(𝑛,C). We
will find parabolic subalgebras giving rise to the partial flag varieties
of C^{𝑛}.

Let us choose a basis for 𝔥 given by matrices ℎ_{𝑖} with components
(ℎ_{𝑖})𝑝 𝑞 =𝛿_{𝑖 𝑝}𝛿_{𝑖 𝑞}−𝛿_{𝑖+}_{1}_{𝑝}𝛿_{𝑖+}_{1}_{𝑞} for 1≤ 𝑖 ≤ 𝑛−1 (all entries are zero except
for two consecutive entries on the diagonal, which are 1 and -1) and let
us complete it to a basis of 𝔤 by adding the matrices 𝑚_{𝑖 𝑗} with
compo-nents (𝑚_{𝑖 𝑗 𝑝}𝑞) = 𝛿_{𝑖 𝑝}𝛿_{𝑗 𝑞} for 1 ≤ 𝑖 ≠ 𝑗 ≤ 𝑛 (i.e. all entries are zero but

the entry𝑖 𝑗, which is equal to one). If we now call ℎ =diag(𝑡_{1}, . . . , 𝑡_{𝑛})
we have

[ℎ, 𝑚_{𝑖 𝑗}] =(𝑡_{𝑖}−𝑡_{𝑗})𝑚_{𝑖 𝑗}

and this defines the roots expanded on the standard basis of 𝑀_{𝑛}_{×}_{𝑛},
in fact, given an element ℎ_{𝑖} of the standard basis of 𝔥, by adapting
Equation 2.2.2 to our double-index notation we write:

[ℎ_{𝑖}, 𝑚_{𝑗 𝑘}] =(𝛿_{𝑖 𝑗} −𝛿_{𝑖 𝑘})𝑚_{𝑗 𝑘} =𝛼_{𝑗 𝑘}(ℎ_{𝑖})𝑚_{𝑗 𝑘}.
Observe that 𝛼_{𝑗 𝑘} =−𝛼_{𝑘 𝑗}.

For simplicity of notation, let us fix 𝑛 = 5. Then, let us consider the following relation: to off-diagonal entries above the first upper-diagonal line. This is made clearer by the following picture:

where the dots represent positive roots which are not simple roots.

Similarly, the negative roots correspond to entries below the diagonal.

We can now characterize the Borel subalgebra: By Equation 2.2.14 we find that 𝔟 is the algebra of upper triangular traceless matrices, be-cause the direct sum of the root spaces corresponding to positive roots is the subspace of strictly upper triangular matrices. Before proceed-ing with the characterization of parabolic subgroups, let us discuss the geometry related to the group 𝐵. Exponentiating 𝔟 we obtain

𝐵 ={𝑏 ∈𝑆 𝐿(𝑛),upper triangular}

Then, 𝐵 clearly acts on 𝑆 𝐿(𝑛) by 𝑔, 𝑏 −→ 𝑔 𝑏^{−}^{1} where the inverse
is to preserve associativity (the choice of right inverse multiplication
insetead of left multiplication is for further convenience). This action
sends the𝑖-th column of𝑔 to a linear combination of the first𝑖 columns.

Clearly, an equivalence class is a chain of subspace of dimensions rang-ing from 1 to 5. Hence, with respect to this action, 𝐺/𝐵 is the variety of complete flags 𝐹(1,2,3,4,5).

Now let us construct the parabolic subalgebra 𝔭_{23}. The first step is
to construct the Levi subalgebra: By Equation 2.2.15 we learn that 𝔩_{23}
is the direct sum of 𝔥 with the span of the first and the fourth simple

roots. We get:

where the bullets correspond to the only entries which are allowed to
be nonzero. To get 𝔭_{23} we need to add up the nilpotent subalgebra,
which is given by the sum of the root spaces corresponding to positive
root which are not appearing in Δ_{23} Pictorially, adding 𝔫_{23} correspond
to "filling up the upper diagonal part". In other words, we get:

𝔭_{23} =

Clearly, the action of matrices of this shape on elements of 𝑆 𝐿(5) preserves the pairs of nested subspaces given respectively by the span of the first two and the first three columns. Therefore, the quotient of 𝑆 𝐿(5) by this action is isomorphic to the flag variety 𝐹(2,3,5).