A **representation** of a Lie algebra
g
is a Lie algebra homomorphism

ρ
:
g
→
g
l
(
V
)
from
g
to the Lie algebra of endomorphisms on a vector space *V* (with the commutator as the Lie bracket), sending an element *x* of
g
to an element *ρ*_{x} of
g
l
(
V
)
.

Explicitly, this means that

ρ
[
x
,
y
]
=
[
ρ
x
,
ρ
y
]
=
ρ
x
ρ
y
−
ρ
y
ρ
x

for all *x,y* in
g
. The vector space *V*, together with the representation ρ, is called a **
g
-module**. (Many authors abuse terminology and refer to *V* itself as the representation).

The representation
ρ
is said to be **faithful** if it is injective.

One can equivalently define a
g
-module as a vector space *V* together with a bilinear map
g
×
V
→
V
such that

[
x
,
y
]
⋅
v
=
x
⋅
(
y
⋅
v
)
−
y
⋅
(
x
⋅
v
)
for all *x,y* in
g
and *v* in *V*. This is related to the previous definition by setting *x* ⋅ *v* = ρ_{x} (v).

The most basic example of a Lie algebra representation is the adjoint representation of a Lie algebra
g
on itself:

ad
:
g
→
g
l
(
g
)
,
x
↦
ad
x
,
ad
x
(
y
)
=
[
x
,
y
]
.
Indeed, by virtue of the Jacobi identity,
ad
is a Lie algebra homomorphism.

A Lie algebra representation also arises in nature. If φ: *G* → *H* is a homomorphism of (real or complex) Lie groups, and
g
and
h
are the Lie algebras of *G* and *H* respectively, then the differential
d
e
ϕ
:
g
→
h
on tangent spaces at the identities is a Lie algebra homomorphism. In particular, for a finite-dimensional vector space *V*, a representation of Lie groups

ϕ
:
G
→
G
L
(
V
)

determines a Lie algebra homomorphism

d
ϕ
:
g
→
g
l
(
V
)
from
g
to the Lie algebra of the general linear group GL(*V*), i.e. the endomorphism algebra of *V*.

For example, let
c
g
(
x
)
=
g
x
g
−
1
. Then the differential of
c
g
:
G
→
G
at the identity is an element of
G
L
(
g
)
. Denoting it by
Ad
(
g
)
one obtains a representation
Ad
of *G* on the vector space
g
. This is the adjoint representation of *G*. Applying the preceding, one gets the Lie algebra representation
d
Ad
. It can be shown that
d
e
Ad
=
ad
, the adjoint representation of
g
.

A partial converse to this statement says that every representation of a finite-dimensional (real or complex) Lie algebra lifts to a unique representation of the associated simply connected Lie group, so that representations of simply-connected Lie groups are in one-to-one correspondence with representations of their Lie algebras.

Let
g
be a Lie algebra. Let *V*, *W* be
g
-modules. Then a linear map
f
:
V
→
W
is a homomorphism of
g
-modules if it is
g
-equivariant; i.e.,
f
(
x
v
)
=
x
f
(
v
)
for any
x
∈
g
,
v
∈
V
. If *f* is bijective,
V
,
W
are said to be *equivalent*. Similarly, many other constructions from module theory in abstract algebra carry over to this setting: submodule, quotient, subquotient, direct sum, Jordan-Hölder series, etc.

Let *V* be a
g
-module. Then *V* is said to be *semisimple* or *completely reducible* if it satisfies the following equivalent conditions: (cf. semisimple module)

*V* is a direct sum of simple modules.
*V* is the sum of its simple submodules.
- Every submodule of
*V* is a direct summand: for every submodule *W* of *V*, there is a complement *P* such that *V* = *W* ⊕ *P*.

If
g
is a finite-dimensional semisimple Lie algebra over a field of characteristic zero and *V* is finite-dimensional, then *V* is semisimple (Weyl's complete reducibility theorem). A Lie algebra is said to be reductive if the adjoint representation is semisimple. Thus, a semisimple Lie algebra is reductive. An element *v* of *V* is said to be
g
-invariant if
x
v
=
0
for all
x
∈
g
. The set of all invariant elements is denoted by
V
g
.
V
↦
V
g
is a left-exact functor.

If we have two representations, with *V*_{1} and *V*_{2} as their underlying vector spaces and ·[·]_{1} and ·[·]_{2} as the representations, then the product of both representations would have *V*_{1} ⊗ *V*_{2} as the underlying vector space and

x
[
v
1
⊗
v
2
]
=
x
[
v
1
]
⊗
v
2
+
v
1
⊗
x
[
v
2
]
.
If *L* is a real Lie algebra and ρ: *L* × *V*→ *V* is a complex representation of it, we can construct another representation of *L* called its dual representation as follows.

Let *V*^{∗} be the dual vector space of *V*. In other words, *V*^{∗} is the set of all linear maps from *V* to **C** with addition defined over it in the usual linear way, but scalar multiplication defined over it such that
(
z
ω
)
[
X
]
=
z
¯
ω
[
X
]
for any *z* in **C**, ω in *V*^{∗} and *X* in *V*. This is usually rewritten as a contraction with a sesquilinear form 〈·,·〉. i.e. 〈ω,*X*〉 is defined to be ω[*X*].

We define
ρ
¯
as follows:

〈

ρ
¯
(

*A*)[ω],

*X*〉 + 〈ω, ρ

*A*[

*X*]〉 = 0,

for any *A* in *L*, ω in *V*^{∗} and *X* in *V*. This defines
ρ
¯
uniquely.

Let
V
,
W
be
g
-modules,
g
a Lie algebra. Then
Hom
(
V
,
W
)
becomes a
g
-module by setting
(
x
⋅
f
)
(
v
)
=
x
f
(
v
)
−
f
(
x
v
)
. In particular,
Hom
g
(
V
,
W
)
=
Hom
(
V
,
W
)
g
. Since any field becomes a
g
-module with a trivial action, taking *W* to be the base field, the dual vector space
V
∗
becomes a
g
-module.

To each Lie algebra
g
over a field *k*, one can associate a certain ring called the universal enveloping algebra of
g
. The construction is universal and consequently (along with the PBW theorem) representations of
g
corresponds in one-to-one with algebra representations of universal enveloping algebra of
g
. The construction is as follows. Let *T* be the tensor algebra of the vector space
g
. Thus, by definition,
T
=
⊕
n
=
0
∞
⊗
1
n
g
and the multiplication on it is given by
⊗
. Let
U
(
g
)
be the quotient ring of *T* by the ideal generated by elements
[
x
,
y
]
−
x
⊗
y
+
y
⊗
x
. Since
U
(
g
)
is an associative algebra over the field *k*, it can be turned into a Lie algebra via the commutator
[
x
,
y
]
=
x
y
−
y
x
(omitting
⊗
from the notation). There is a canonical morphism of Lie algebras
g
→
U
(
g
)
obtained by restricting
T
→
U
(
g
)
to degree one piece. The PBW theorem implies that the canonical map is actually injective. Note if
g
is abelian, then
U
(
g
)
is the symmetric algebra of the vector space
g
.

Since
g
is a module over itself via adjoint representation, the enveloping algebra
U
(
g
)
becomes a
g
-module by extending the adjoint representation. But one can also use the left and right regular representation to make the enveloping algebra a
g
-module; namely, with the notation
l
x
(
y
)
=
x
y
,
x
∈
g
,
y
∈
U
(
g
)
, the mapping
x
↦
l
x
defines a representation of
g
on
U
(
g
)
. The right regular representation is defined similarly.

Let
g
be a finite-dimensional Lie algebra over a field of characteristic zero and
h
⊂
g
a subalgebra.
U
(
h
)
acts on
U
(
g
)
from the right and thus, for any
h
-module *W*, one can form the left
U
(
g
)
-module
U
(
g
)
⊗
U
(
h
)
W
. It is a
g
-module denoted by
Ind
h
g
W
and called the
g
-module induced by *W*. It satisfies (and is in fact characterized by) the universal property: for any
g
-module *E*

Hom
g
(
Ind
h
g
W
,
E
)
≃
Hom
h
(
W
,
Res
h
g
E
)
.

Furthermore,
Ind
h
g
is an exact functor from the category of
h
-modules to the category of
g
-modules. These uses the fact that
U
(
g
)
is a free right module over
U
(
h
)
. In particular, if
Ind
h
g
W
is simple (resp. absolutely simple), then *W* is simple (resp. absolutely simple). Here, a
g
-module *V* is absolutely simple if
V
⊗
k
F
is simple for any field extension
F
/
k
.

The induction is transitive:
Ind
h
g
≃
Ind
h
′
g
∘
Ind
h
h
′
for any Lie subalgebra
h
′
⊂
g
and any Lie subalgebra
h
⊂
h
′
. The induction commutes with restriction: let
h
⊂
g
be subalgebra and
n
an ideal of
g
that is contained in
h
. Set
g
1
=
g
/
n
and
h
1
=
h
/
n
. Then
Ind
h
g
∘
Res
h
≃
Res
g
∘
Ind
h
1
g
1
.

Let
g
be a finite-dimensional semisimple Lie algebra over a field of characteristic zero. (in the solvable or nilpotent case, one studies primitive ideals of the enveloping algebra; cf. Dixmier for the definitive account.)

The category of modules over
g
turns out to be too large especially for homological algebra methods to be useful: it was realized that a smaller subcategory category O is a better place for the representation theory in the semisimple case in zero characteristic. For instance, the category O turned out to be of a right size to formulate the celebrated BGG reciprocity.

One of the most important applications of Lie algebra representations is to the representation theory of real reductive Lie group. The application is based on the idea that if
π
is a Hilbert-space representation of, say, a connected real semisimple linear Lie group *G*, then it has two natural actions: the complexification
g
and the connected maximal compact subgroup *K*. The
g
-module structure of
π
allows algebraic especially homological methods to be applied and
K
-module structure allows harmonic analysis to be carried out in a way similar to that on connected compact semisimple Lie groups.

Similarly to how semisimple Lie algebras can be classified, the finite-dimensional representations of semisimple Lie algebras can be classified. This is a beautiful, classical theory, described in several textbooks, including (Fulton & Harris 1992), (Hall 2015), and (Humphreys 1972).

Briefly, finite-dimensional representations of a semisimple Lie algebra are completely reducible, so it suffices to classify irreducible (simple) representations. The irreducible representations, in turn, may be classified by the "theorem of the highest weight." The theorem states that every irreducible representation has a dominant integral highest weight, two irreducible representations with the same highest weight are isomorphic, and that every dominant integral element occurs as the highest weight of some irreducible representation. The last point is the most difficult one; construction of the representations may be given by using Verma modules. This classification generalizes the more elementary representation theory of sl(2;C), where the irreducible representations are classified by the largest eigenvalue of the diagonal element *H*.

If we have a Lie superalgebra *L*, then a representation of *L* on an algebra is a (not necessarily associative) **Z**_{2} graded algebra *A* which is a representation of *L* as a **Z**_{2} graded vector space and in addition, the elements of *L* acts as derivations/antiderivations on *A*.

More specifically, if *H* is a pure element of *L* and *x* and *y* are pure elements of *A*,

*H*[

*xy*] = (

*H*[

*x*])

*y* + (−1)

^{xH}*x*(

*H*[

*y*])

Also, if *A* is unital, then

*H*[1] = 0

Now, for the case of a **representation of a Lie algebra**, we simply drop all the gradings and the (−1) to the some power factors.

A Lie (super)algebra is an algebra and it has an adjoint representation of itself. This is a representation on an algebra: the (anti)derivation property is the superJacobi identity.

If a vector space is both an associative algebra and a Lie algebra and the adjoint representation of the Lie algebra on itself is a representation on an algebra (i.e., acts by derivations on the associative algebra structure), then it is a Poisson algebra. The analogous observation for Lie superalgebras gives the notion of a Poisson superalgebra.