On Free Pseudo-Product Fundamental Graded Lie Algebras
In this paper we first state the classification of the prolongations of complex free fundamental graded Lie algebras. Next we introduce the notion of free pseudo-product fundamental graded Lie algebras and study the prolongations of complex free pseudo-product fundamental graded Lie algebras. Furthe...
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irk-123456789-1484452019-02-19T01:23:13Z On Free Pseudo-Product Fundamental Graded Lie Algebras Yatsui, T. In this paper we first state the classification of the prolongations of complex free fundamental graded Lie algebras. Next we introduce the notion of free pseudo-product fundamental graded Lie algebras and study the prolongations of complex free pseudo-product fundamental graded Lie algebras. Furthermore we investigate the automorphism group of the prolongation of complex free pseudo-product fundamental graded Lie algebras. 2012 Article On Free Pseudo-Product Fundamental Graded Lie Algebras / T. Yatsui // Symmetry, Integrability and Geometry: Methods and Applications. — 2012. — Т. 8. — Бібліогр.: 16 назв. — англ. 1815-0659 2010 Mathematics Subject Classification: 17B70 DOI: http://dx.doi.org/10.3842/SIGMA.2012.038 http://dspace.nbuv.gov.ua/handle/123456789/148445 en Symmetry, Integrability and Geometry: Methods and Applications Інститут математики НАН України |
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In this paper we first state the classification of the prolongations of complex free fundamental graded Lie algebras. Next we introduce the notion of free pseudo-product fundamental graded Lie algebras and study the prolongations of complex free pseudo-product fundamental graded Lie algebras. Furthermore we investigate the automorphism group of the prolongation of complex free pseudo-product fundamental graded Lie algebras. |
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Yatsui, T. On Free Pseudo-Product Fundamental Graded Lie Algebras Symmetry, Integrability and Geometry: Methods and Applications |
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On Free Pseudo-Product Fundamental Graded Lie Algebras |
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On Free Pseudo-Product Fundamental Graded Lie Algebras |
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On Free Pseudo-Product Fundamental Graded Lie Algebras |
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On Free Pseudo-Product Fundamental Graded Lie Algebras |
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On Free Pseudo-Product Fundamental Graded Lie Algebras |
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on free pseudo-product fundamental graded lie algebras |
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Інститут математики НАН України |
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On Free Pseudo-Product Fundamental Graded Lie Algebras / T. Yatsui // Symmetry, Integrability and Geometry: Methods and Applications. — 2012. — Т. 8. — Бібліогр.: 16 назв. — англ. |
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Symmetry, Integrability and Geometry: Methods and Applications |
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AT yatsuit onfreepseudoproductfundamentalgradedliealgebras |
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2025-07-12T19:29:27Z |
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Symmetry, Integrability and Geometry: Methods and Applications SIGMA 8 (2012), 038, 18 pages
On Free Pseudo-Product
Fundamental Graded Lie Algebras
Tomoaki YATSUI
Department of Mathematics, Asahikawa Medical University, Asahikawa 078-8510, Japan
E-mail: yatsui@asahikawa-med.ac.jp
Received December 16, 2011, in final form June 14, 2012; Published online June 27, 2012
http://dx.doi.org/10.3842/SIGMA.2012.038
Abstract. In this paper we first state the classification of the prolongations of complex
free fundamental graded Lie algebras. Next we introduce the notion of free pseudo-product
fundamental graded Lie algebras and study the prolongations of complex free pseudo-product
fundamental graded Lie algebras. Furthermore we investigate the automorphism group of
the prolongation of complex free pseudo-product fundamental graded Lie algebras.
Key words: fundamental graded Lie algebra; prolongation; pseudo-product graded Lie alge-
bra
2010 Mathematics Subject Classification: 17B70
1 Introduction
Let m =
⊕
p<0
gp be a graded Lie algebra over the field R of real numbers or the field C of
complex numbers, and let µ be a positive integer. The graded Lie algebra m =
⊕
p<0
gp is called
a fundamental graded Lie algebra if the following conditions hold: (i) m is finite-dimensional;
(ii) g−1 6= {0}, and m is generated by g−1. Moreover a fundamental graded Lie algebra m =⊕
p<0
gp is said to be of the µ-th kind if g−µ 6= {0}, and gp = {0} for all p < −µ. It is shown
that every fundamental graded algebra m =
⊕
p<0
gp is prolonged to a graded Lie algebra g(m) =⊕
p∈Z
g(m)p satisfying the following conditions: (i) g(m)p = gp for all p < 0; (ii) for X ∈ g(m)p
(p = 0), [X,m] = {0} implies X = 0; (iii) g(m) is maximum among graded Lie algebras satisfying
conditions (i) and (ii) above. The graded Lie algebra g(m) is called the prolongation of m. Note
that g(m)0 is the Lie algebra of all the derivations of m as a graded Lie algebra.
Let m =
⊕
p<0
gp be a fundamental graded Lie algebra of the µ-th kind, where µ = 2. The
fundamental graded Lie algebra m is called a free fundamental graded Lie algebra of type (n, µ)
if the following universal properties hold:
(i) dim g−1 = n;
(ii) Let m′ =
⊕
p<0
g′p be a fundamental graded Lie algebra of the µ-th kind and let ϕ be
a surjective linear mapping of g−1 onto g′−1. Then ϕ can be extended uniquely to a graded
Lie algebra epimorphism of m onto m′.
In Section 3 we see that a universal fundamental graded Lie algebra b(V, µ) of the µ-th kind
introduced by N. Tanaka [11] becomes a free fundamental graded Lie algebra of type (n, µ),
where µ = 2, and V is a vector space such that dimV = n = 2.
mailto:yatsui@asahikawa-med.ac.jp
http://dx.doi.org/10.3842/SIGMA.2012.038
2 T. Yatsui
In [13], B. Warhurst gave the complete list of the prolongations of real free fundamental
graded Lie algebras by using a Hall basis of a free Lie algebra. The complex version of his
theorem has the completely same form except for the ground number field as follows:
Theorem I. Let m =
⊕
p<0
gp be a free fundamental graded Lie algebra of type (n, µ) over C.
Then the prolongation g(m) =
⊕
p∈Z
g(m)p of m is one of the following types:
(a) (n, µ) 6= (n, 2) (n = 2), (2, 3). In this case, g(m)1 = {0}.
(b) (n, µ) = (n, 2) (n = 3), (2, 3). In this case, dim g(m) <∞ and g(m)1 6= {0}. Furthermore
g(m) is isomorphic to a finite-dimensional simple graded Lie algebra of type (Bn, {αn})
(n = 3) or (G2, {α1}) (n = 2) (see [15] or Section 5 for the gradations of finite-dimensional
simple graded Lie algebras over C).
(c) (n, µ) = (2, 2). In this case, dim g(m) = ∞. Furthermore, g(m) is isomorphic to the
contact algebra K(1) as a graded Lie algebra.
The first purpose of this paper is to give a proof of Theorem I by using the classification of
complex irreducible transitive graded Lie algebras of finite depth (cf. [6]). Note that Warhurst’s
methods in [13] are available to the proof of Theorem I.
Next we introduce the notion of free pseudo-product fundamental graded Lie algebras. Let
m =
⊕
p<0
gp be a fundamental graded Lie algebra, and let e and f be nonzero subspaces of g−1.
Then m is called a pseudo-product fundamental graded Lie algebra with pseudo-product struc-
ture (e, f) if the following conditions hold: (i) g−1 = e⊕ f; (ii) [e, e] = [f, f] = {0} (cf. [10]).
Let m =
⊕
p<0
gp be a pseudo-product fundamental graded Lie algebra with a pseudo-product
structure (e, f), and let g(m) =
⊕
p∈Z
g(m)p be the prolongation of m. Moreover let g0 be the Lie
algebra of all the derivations of m as a graded Lie algebra preserving e and f. Also for p = 1 we
set gp = {X ∈ g(m)p : [X, gk] ⊂ gp+k for all k < 0} inductively. Then the direct sum g =
⊕
p∈Z
gp
becomes a graded subalgebra of g(m), which is called the prolongation of (m; e, f).
Let m =
⊕
p<0
gp be a pseudo-product fundamental graded Lie algebra of the µ-th kind with
pseudo-product structure (e, f), where µ = 2. The pseudo-product fundamental graded Lie
algebra m =
⊕
p<0
gp is called a free pseudo-product fundamental graded Lie algebra of type
(m,n, µ) if the following conditions hold:
(i) dim e = m and dim f = n;
(ii) Let m′ =
⊕
p<0
g′p be a pseudo-product fundamental graded Lie algebra of the µ-th kind with
pseudo-product structure (e′, f′) and let ϕ be a surjective linear mapping of g−1 onto g′−1
such that ϕ(e) ⊂ e′ and ϕ(f) ⊂ f′. Then ϕ can be extended uniquely to a graded Lie
algebra epimorphism of m onto m′.
The main purpose of this paper is to prove the following theorem.
Theorem II. Let m =
⊕
p<0
gp be a free pseudo-product fundamental graded Lie algebra of type
(m,n, µ) with pseudo-product structure (e, f) over C, and let g =
⊕
p∈Z
gp be the prolongation of
(m; e, f). If g1 6= {0}, then g =
⊕
p∈Z
gp is a finite-dimensional simple graded Lie algebra of type
(Am+n, {αm, αm+1}).
On Free Pseudo-Product Fundamental Graded Lie Algebras 3
Let g =
⊕
p∈Z
gp be the prolongation of a free pseudo-product fundamental graded Lie algebra
m =
⊕
p<0
gp with pseudo-product structure (e, f) over C. We denote by Aut(g; e, f)0 the group of
all the automorphisms as a graded Lie algebra preserving e and f, which is called the automor-
phism group of the pseudo-product graded Lie algebra g =
⊕
p∈Z
gp. In Section 9, we show that
Aut(g; e, f)0 is isomorphic to GL(e)×GL(f).
Notation and conventions
(1) From Section 2 to the last section, all vector spaces are considered over the field C of
complex numbers.
(2) Let V be a vector space and let W1 and W2 be subspaces of V . We denote by W1 ∧W2
the subspace of Λ2V spanned by all the elements of the form w1∧w2 (w1 ∈W1, w2 ∈W2).
(3) Graded vector spaces are always Z-graded. If we write V =
⊕
p<0
Vp, then it is understood
that Vp = {0} for all p = 0. Let V =
⊕
p∈Z
Vp be a graded vector space. We denote by V−
the subspace V =
⊕
p<0
Vp. Also for k ∈ Z we denote by V5k the subspace
⊕
p5k
Vp.
Let V =
⊕
p∈Z
Vp and W =
⊕
p∈Z
Wp be graded vector spaces. For r ∈ Z, we set
Hom(V,W )r = {ϕ ∈ Hom(V,W ) : ϕ(Vp) ⊂Wp+r for all p ∈ Z}.
2 Free fundamental graded Lie algebras
First of all we give several definitions about graded Lie algebras. Let g be a Lie algebra. Assume
that there is given a family of subspaces (gp)p∈Z of g satisfying the following conditions:
(i) g =
⊕
p∈Z
gp;
(ii) dim gp <∞ for all p ∈ Z;
(iii) [gp, gq] ⊂ gp+q for all p, q ∈ Z.
Under these conditions, we say that g =
⊕
p∈Z
gp is a graded Lie algebra (GLA). Moreover we
define the notion of homomorphism, isomorphism, monomorphism, epimorphism, subalgebra
and ideal for GLAs in an obvious manner.
A GLA g =
⊕
p∈Z
gp is called transitive if for X ∈ gp (p = 0), [X, g−] = {0} implies X = 0,
where g− is the negative part
⊕
p<0
gp of g. Furthermore a GLA g =
⊕
p∈Z
gp is called irreducible if
the g0-module g−1 is irreducible.
Let µ be a positive integer. A GLA g =
⊕
p∈Z
gp is said to be of depth µ if g−µ 6= {0} and
gp = {0} for all p < −µ.
Next we define fundamental GLAs. A GLA m =
⊕
p<0
gp is called a fundamental graded Lie
algebra (FGLA) if the following conditions hold:
(i) dimm <∞;
(ii) g−1 6= {0}, and m is generated by g−1, or more precisely gp−1 = [gp, g−1] for all p < 0.
4 T. Yatsui
If an FGLA m =
⊕
p<0
gp is of depth µ, then m is also said to be of the µ-th kind. Moreover an
FGLA m =
⊕
p<0
gp is called non-degenerate if for X ∈ g−1, [X, g−1] = {0} implies X = 0.
Let m =
⊕
p<0
gp be an FGLA of the µ-th kind, where µ = 2. m is called a free fundamental
graded Lie algebra of type (n, µ) if the following conditions hold:
(i) dim g−1 = n;
(ii) Let m′ =
⊕
p<0
g′p be an FGLA of the µ-th kind and let ϕ be a surjective linear mapping of
g−1 onto g′−1. Then ϕ can be extended uniquely to a GLA epimorphism of m onto m′.
Proposition 2.1. Let n and µ be positive integers such that n, µ = 2.
(1) There exists a unique free FGLA of type (n, µ) up to isomorphism.
(2) Let m =
⊕
p<0
gp be a free FGLA of type (n, µ). We denote by Der(m)0 the Lie algebra of
all the derivations of m preserving the gradation of m. Then the mapping Φ : Der(m)0 3
D 7→ D|g−1 ∈ gl(g−1) is a Lie algebra isomorphism.
Proof. (1) The uniqueness of a free FGLA of type (n, µ) follows from the definition. We set
X = {1, . . . , n}. Let L(X) be the free Lie algebra on X (see [1, Chapter II, § 2]) and let
i : X → L(X) be the canonical injection. We define a mapping φ of X into Z by φ(k) = −1
(k ∈ X). The mapping φ defines the natural gradation (L(X)p)p<0 on L(X) such that: (i)
L(X) is generated by L(X)−1; (ii) {i(1), . . . , i(n)} is a basis of L(X)−1 (see [1, Chapter II, § 2,
no. 6]). Note that if n > 1, then L(X)p 6= 0 for all p < 0. We set a =
⊕
p<−µ
L(X)p; then a is a
graded ideal of L(X) and the factor GLA m = L(X)/a becomes an FGLA of the µ-th kind. We
put ap = a ∩ L(X)p and gp = L(X)p/ap.
Now we prove that m =
⊕
p<0
gp is a free FGLA of type (n, µ). Let m′ =
⊕
p<0
g′p be an FGLA of
the µ-th kind and let ϕ be a surjective linear mapping of g−1 onto g′−1. Let h be a mapping of X
into m′ defined by h(k) = ϕ(i(k)) (k ∈ X). Then there exists a Lie algebra homomorphism h̃
of L(X) into m′ such that h̃◦i = h. Since L(X) (resp. m′) is generated by L(X)−1 (resp. g′−1), h̃ is
surjective. Since m′ =
⊕
p<0
g′p is of the µ-th kind, h̃(a) = 0, so h̃ induces a GLA epimorphism L(ϕ)
of m onto m′ such that L(ϕ)|g−1 = ϕ. The homomorphism L(ϕ) is unique, because m =
⊕
p<0
gp
is generated by g−1. Thus m is a free FGLA of type (n, µ).
(2) Assume that m is a free FGLA constructed in (1). Let φ be an endomorphism of g−1.
By Corollary to Proposition 8 of [1, Chapter II, § 2, no. 8], φ can be extended uniquely to
a unique derivation D of L(X). Since D(L(X)−1) = φ(L(X)−1) = φ(g−1) ⊂ L(X)−1, and
since L(X) is generated by L(X)−1, we see that D(L(X)p) ⊂ L(X)p and D(a) ⊂ a. Thus there
is a derivation of Dφ of m such that π ◦D = Dφ ◦ π, where π is the natural projection of L(X)
onto m. The correspondence gl(g−1) 3 φ 7→ Dφ ∈ Der(m)0 is an injective linear mapping. Hence
dim gl(g−1) 5 dim Der(m)0. On the other hand, since m is generated by g−1, the mapping Φ is
a Lie algebra monomorphism. Therefore Φ is a Lie algebra isomorphism. �
Remark 2.1. Let n and µ be positive integers with n, µ = 2, and let m =
⊕
p<0
gp be a free FGLA
of type (n, µ). Furthermore let m′ =
⊕
p<0
g′p be an FGLA of the µ-th kind, and let ϕ be a linear
mapping of g−1 into g′−1.
(1) From the proof of Proposition 2.1, there exists a unique GLA homomorphism L(ϕ) of m
into m′ such that L(ϕ)|g−1 = ϕ.
On Free Pseudo-Product Fundamental Graded Lie Algebras 5
(2) Let m′′ =
⊕
p<0
g′′p be an FGLA of the µ-th kind, and let ϕ′ be a linear mapping of g′−1
into g′′−1. Assume that m′ =
⊕
p<0
g′p is a free FGLA. By the uniqueness of L(ϕ′ ◦ ϕ), we see
that L(ϕ′ ◦ ϕ) = L(ϕ′) ◦ L(ϕ).
(3) Assume that m′ =
⊕
p<0
g′p is a free FGLA and ϕ is injective. By the result of (2), L(ϕ) is
a monomorphism.
(4) Let W be an m-dimensional subspace of g−1 with m = 2. By the result of (3), the
subalgebra of m generated by W is a free FGLA of type (m,µ).
By Remark 2.1 (4) and [1, Chapter II, § 2, Theorem 1], we get the following lemma.
Lemma 2.1. Let m =
⊕
p<0
gp be a free FGLA of type (n, µ) with µ = 3. If X, Y are linearly
independent elements of g−1, then
ad(X)µ(Y ) = 0, ad(X)µ−1(Y ) 6= 0,
ad(Y ) ad(X)µ−1(Y ) = 0, ad(Y ) ad(X)µ−2(Y ) 6= 0.
3 Universal fundamental graded Lie algebras
Following N. Tanaka [11], we introduce universal FGLAs of the µ-th kind.
Let V be an n-dimensional vector space. We define vector spaces b(V )p (p < 0) and li-
near mappings Bp of
∑
r+s=p
b(V )r ∧ b(V )s into b(V )p (p 5 −2) as follows: First of all, we put
b(V )−1 = V and b(V )−2 = Λ2V . Further we define a mapping B−2 : b(V )−1∧ b(V )−1 → b(V )−2
to be the identity mapping. For k 5 −3, we define b(V )k and Bk inductively as follows: We
set b(V )(k+1) =
k+1⊕
p=−1
b(V )p and we define a subspace c(V )k of Λ2(b(V )(k+1)) to be
∑
r+s=k
b(V )r ∧
b(V )s. We denote by A(V )k the subspace of c(V )k spanned by the elements
S
(X,Y,Z)
∑
r+s=k
∑
u+v=r
Br(Xu ∧ Yv) ∧ Zs, X, Y, Z ∈ b(V )(k+1),
where S
(X,Y,Z)
stands for the cyclic sum with respect to X, Y , Z, and Xu denotes the b(V )u-
component in the decomposition b(V )(k+1) =
k+1⊕
p=−1
b(V )p. Now we define b(V )k to be the factor
space c(V )k/A(V )k, and Bk to be the projection of c(V )k onto b(V )k. We put b(V ) =
⊕
p<0
b(V )p
and define a bracket operation [ , ] on b(V ) by
[X,Y ] =
∑
p5−2
∑
r+s=p
Bp(Xr ∧ Ys)
for all X,Y ∈ b(V ). Then b(V ) =
⊕
p<0
b(V )p becomes a GLA generated by b(V )−1, and b(V )p 6= 0
for all p < 0 if dimV > 1.
Note that b(V )−3 is isomorphic to Λ2(V )⊗V/Λ3V . Let µ be a positive integer. Assume that
µ = 2 and dimV = n = 2. Since
⊕
p<−µ
b(V )p is a graded ideal of b(V ), we see that the factor
space b(V, µ) = b(V )/
⊕
p<−µ
b(V )p becomes an FGLA of µ-th kind, which is called a universal
fundamental graded Lie algebra of the µ-th kind. By [11, Proposition 3.2], b(V, µ) is a free
FGLA of type (n, µ).
6 T. Yatsui
4 The prolongations of fundamental graded Lie algebras
Following N. Tanaka [11], we introduce the prolongations of FGLAs. Let m =
⊕
p<0
gp be an
FGLA. A GLA g(m) =
⊕
p∈Z
g(m)p is called the prolongation of m if the following conditions hold:
(i) g(m)p = gp for all p < 0;
(ii) g(m) is a transitive GLA;
(iii) If h =
⊕
p∈Z
hp is a GLA satisfying conditions (i) and (ii) above, then h =
⊕
p∈Z
hp can be
embedded in g(m) as a GLA.
We construct the prolongation g(m) =
⊕
p∈Z
g(m)p of m. We set g(m)p = gp (p < 0). We define
subspaces g(m)k (k = 0) of Hom(m,
⊕
p5k−1
g(m)p)k and a bracket operation on g(m) =
⊕
p∈Z
g(m)p
inductively. First g(m)0 is defined to be Der(m)0 and a bracket operation [ , ] :
⊕
p50
g(m)p ×⊕
p50
g(m)p →
⊕
p50
g(m)p is defined by
[X,Y ] = −[Y,X] = X(Y ), X ∈ g(m)0, Y ∈ m,
[X,Y ] = XY − Y X, X, Y ∈ g(m)0.
Next for k > 0 we define g(m)k (k = 1) inductively as follows:
g(m)k =
{
X ∈ Hom
(
m,
⊕
p5k−1
g(m)p
)
k
: X([u, v]) = [X(u), v] + [u,X(v)] for all u, v ∈ m
}
,
where for X ∈ g(m)r, u ∈ m, we set [X,u] = −[u,X] = X(u). Further for X ∈ g(m)k, Y ∈ g(m)l
(k, l = 0), by induction on k + l = 0, we define [X,Y ] ∈ Hom(m, g(m))k+l by
[X,Y ](u) = [X, [Y, u]]− [Y, [X,u]], u ∈ m.
It follows easily that [X,Y ] ∈ g(m)k+l. With this bracket operation, g(m) =
⊕
p∈Z
g(m)p becomes
a graded Lie algebra satisfying conditions (i), (ii) and (iii) above.
Let m and g(m) be as above. Assume that we are given a subalgebra g0 of g(m)0. We define
subspaces gk (k = 1) of g(m)k inductively as follows:
gk = {X ∈ g(m)k : [X, gp] ⊂ gp+k for all p < 0}.
If we put g =
⊕
p∈Z
gp, then g =
⊕
p∈Z
gp becomes a transitive graded Lie subalgebra of g(m), which
is called the prolongation of (m, g0).
By Proposition 2.1 (2) we get the following proposition.
Proposition 4.1. Let m =
⊕
p<0
gp be a free FGLA and let g(m) =
⊕
p∈Z
g(m)p be the prolongation
of m. Then the mapping g(m)0 3 D 7→ D|g−1 ∈ gl(g−1) is an isomorphism.
Conversely we obtain the following proposition.
Proposition 4.2. Let m =
⊕
p<0
gp be an FGLA of the µ-th kind and let g(m) =
⊕
p∈Z
g(m)p be the
prolongation of m. Assume that g(m)0 is isomorphic to gl(g−1). If µ = 2 or µ = 3, then m is
a free FGLA.
On Free Pseudo-Product Fundamental Graded Lie Algebras 7
Proof. We put n = dim g−1. We consider a universal FGLA b(g−1, µ) =
⊕
p<0
b(g−1, µ)p of the
µ-th kind. Since b(g−1, µ) is a free FGLA of type (n, µ), there exists a GLA epimorphism ϕ of
b(g−1, µ) onto m such that the restriction ϕ|b(g−1, µ)−1 is the identity mapping. Let b̌(g−1, µ) =⊕
p∈Z
b̌(g−1, µ)p be the prolongation of b(g−1, µ). Since the mapping g(m)0 3 D 7→ D|g−1 ∈ gl(g−1)
is an isomorphism, ϕ can be extended to be a homomorphism ϕ̌ of
⊕
p50
b̌(g−1, µ)p onto
⊕
p50
g(m)p.
Let a be the kernel of ϕ̌; then a is a graded ideal of
⊕
p50
b̌(g−1, µ)p. We set ap = a∩b̌(g−1, µ)p; then
a =
⊕
p50
ap. Since the restriction of ϕ̌ to b̌(g−1, µ)−1⊕ b̌(g−1, µ)0 is injective, ap = {0} for p = −1.
Also each ap is a b̌(g−1, µ)0-submodule of b̌(g−1, µ)p. From the construction of b(g−1, µ), we see
that b(g−1, µ)−2 (resp. b(g−1, µ)−3) is isomorphic to Λ2(g−1) (resp. Λ2(g−1)⊗ g−1/Λ
3(g−1)) as
a b̌(g−1, µ)0-module. By the table of [8], Λ2(g−1) and Λ2(g−1) ⊗ g−1/Λ
3(g−1) are irreducible
gl(g−1)-modules. Thus we see that a−2 = a−3 = {0}. From µ 5 3 it follows that ϕ is an
isomorphism. �
5 Finite-dimensional simple graded Lie algebras
Following [15], we first state the classification of finite-dimensional simple GLAs.
Let g =
⊕
p∈Z
gp be a finite-dimensional simple GLA of the µ-th kind over C such that the
negative part g− is an FGLA. Let h be a Cartan subalgebra of g0; then h is a Cartan subalgebra
of g such that E ∈ h, where E is the element of g0 such that [E, x] = px for all x ∈ gp and p.
Let ∆ be a root system of (g, h). For α ∈ ∆, we denote by gα the root space corresponding
to α. We set hR = {h ∈ h : α(h) ∈ R for all α ∈ ∆} and let (h1, . . . , hl) be a basis of hR such
that h1 = E. We define the set of positive roots ∆+ as the set of roots which are positive with
respect to the lexicographical ordering in h∗R determined by the basis (h1, . . . , hl) of hR. Let
Π ⊂ ∆+ be the corresponding simple root system. We denote by {m1, . . . ,ml} the coordinate
functions corresponding to Π, i.e., for α ∈ ∆, we can write α =
l∑
i=1
mi(α)αi.
We set αi(E) = si and s = (s1, . . . , sl); then each si is a non-negative integer. For α ∈ ∆,
we call the integer `s(α) =
l∑
i=1
mi(α)si the s-length of α. We put ∆p = {α ∈ ∆ : `s(α) = p},
Πp = ∆p ∩ Π and I = {i ∈ {1, . . . , l} : si = 1}. Let θ be the highest root of g; then `s(θ) = µ.
Also since the g0-module g−µ is irreducible, dim g−µ = 1 if and only if 〈θ, α∨i 〉 = 0 for all
i ∈ {1, . . . , l} \ I, where {α∨i } is the simple root system of the dual root system ∆∨ of ∆
corresponding to {αi}. In our situation, since g− is generated by g−1, we have si = 0 or 1 for
all i. The l-tuple s = (s1, . . . , sl) of non-negative integers is determined only by the ordering of
(α1, . . . , αl). In what follows, we assume that the ordering of (α1, . . . , αl) is as in the table of [2].
If g has the Dynkin diagram of type Xl (X = A, . . . , G), then the simple GLA g =
⊕
p∈Z
gp is said
to be of type (Xl,Π1). Here we remark that for an automorphism µ̄ of the Dynkin diagram,
a simple GLA of type (Xl,Π1) is isomorphic to that of type (Xl, µ̄(Π1)). We will identify a simple
GLA of type (Xl,Π1) with that of type (Xl, µ̄(Π1)).
For i ∈ I, we put ∆
(i)
p = {α ∈ ∆ : mi(α) = p and mj(α) = 0 for j ∈ I \ {i}} and
g
(i)
p =
∑
α∈∆
(i)
p
gα; then g
(i)
−1 is an irreducible g0-submodule of g−1 with highest weight −αi. In
particular, if the g0-module g−1 is irreducible, then #(I) = 1.
8 T. Yatsui
For i ∈ I, we denote by g(i) the subalgebra of g generated by g
(i)
−1 ⊕ g
(i)
1 ; then g(i) is a simple
GLA whose Dynkin diagram is the connected component containing the vertex i of the sub-
diagram of Xl corresponding to vertices ({1, . . . , l} \ I)∪{i}. We denote by θ(i) the highest root
of g(i). Then [g
(i)
−1, g
(i)
−1] = {0} if and only if mi(θ
(i)) = 1.
From Theorem 5.2 of [15], we obtain the following theorem:
Theorem 5.1. Let g =
⊕
p∈Z
gp be a finite-dimensional simple GLA over C such that g− is an
FGLA and the g0-module g−1 is irreducible. Then g =
⊕
p∈Z
gp is the prolongation of g− except
for the following cases:
(a) g− is of the first kind;
(b) g− is of the second kind and dim g−2 = 1.
Let g =
⊕
p∈Z
gp be a finite-dimensional simple GLA. Now we assume that g0 is isomor-
phic to gl(g−1); then the g0-module g−1 is irreducible. The derived subalgebra [g0, g0] of g0
is a semisimple Lie algebra whose Dynkin diagram is the subdiagram of Xl consisting of the
vertices {1, . . . , l} \ I. Since [g0, g0] is of type Al−1 and since the g0-module g−1 is elementary,
(Xl,∆1) is one of the following cases:
(Al, {α1}), (Bl, {αl}), l = 2, (G2, {α1}).
From this result and Propositions 4.1 and 4.2, we get the following theorem:
Theorem 5.2. Let g =
⊕
p∈Z
gp be a finite-dimensional simple GLA of type (Xl,Π1) over C
satisfying the following conditions:
(i) g− is an FGLA of the µ-th kind;
(ii) The g0-module g−1 is irreducible;
(iii) g0 is isomorphic to gl(g−1);
(iv) g is the prolongation of g−.
Then g− is a free FGLA of type (l, µ), and g =
⊕
p∈Z
gp is one of the following types:
(a) l = 3, µ = 2, (Xl,Π1) = (Bl, {αl}).
(b) l = 2, µ = 3, (Xl,Π1) = (G2, {α1}).
6 Graded Lie algebras W (n), K(n) of Cartan type
In this section, following V.G. Kac [3], we describe Lie algebras W (n), K(n) of Cartan type and
their standard gradations.
Let A(m) denote the monoid (under addition) of all m-tuples of non-negative integers. For
an m-tuple s = (s1, . . . , sm) of positive integers and α = (α1, . . . , αm) ∈ A(m) we set ‖α‖s =
m∑
i=1
siαi. Also we denote the m-tuple (1, . . . , 1) by 1m and we denote the (m+1)-tuple (1, . . . , 1, 2)
by (1m, 2). Let A(m) = C[x1, . . . , xm]. For any m-tuple s of positive integers, we denote by
A(m; s)p the subspace of A(m) spanned by polynomials
xα = xα1
1 · · ·x
αm
m , α = (α1, . . . , αm) ∈ A(m), ‖α‖s = p.
On Free Pseudo-Product Fundamental Graded Lie Algebras 9
Let W (m) be the Lie algebra consisting of all the polynomial vector fields
m∑
i=1
Pi
∂
∂xi
, Pi ∈ A(m). (6.1)
For an m-tuple s = (s1, . . . , sm) of positive integers, we denote by W (m; s)p the subspaces
of W (m) consisting of those polynomial vector fields (6.1) such that the polynomials Pi are
contained in A(m; s)p+si ; then W (m; s) =
⊕
p∈Z
W (m; s)p is a transitive GLA. In particular,
W (m;1m) =
⊕
p=−1
W (m;1m)p is a transitive irreducible GLA such that: (i) W (m;1m)0 is iso-
morphic to gl(m,C); (ii) the W (m;1m)0-module W (m;1m)−1 is elementary; (iii) W (m;1m) is
the prolongation of W (m;1m)−.
We now consider the following differential form
ωK = dx2n+1 −
n∑
i=1
xi+ndxi.
Define
K(n) = {D ∈W (2n+ 1) : DωK ∈ A(2n+ 1)ωK}.
(Here the action of D on the differential forms is extended from its action A(2n+1) by requiring
that D be derivation of the exterior algebra satisfying D(df) = d(Df), where df =
∑ ∂f
∂xi
dxi,
f ∈ A(m).) We set K(n)p = W (2n+1; (12n, 2))p∩K(n). Then K(n) =
⊕
p=−2
K(n)p is a transitive
irreducible GLA such that: (i) K(n)0 is isomorphic to csp(n,C); (ii) the K(n)0-module K(n)−1
is elementary; (iii) K(n) is the prolongation of K(n)− (cf. [3, 5]).
From Proposition 2.2 of [6], we get
Theorem 6.1. Let g =
⊕
p∈Z
gp be a transitive GLA over C satisfying the following conditions:
(i) g− is an FGLA of the µ-th kind;
(ii) g is infinite-dimensional;
(iii) The g0-module g−1 is irreducible;
(iv) g is the prolongation of g−.
Then µ 5 2 and g =
⊕
p∈Z
gp is isomorphic to W (m;1m) or K(n).
7 Classif ication of the prolongations
of free fundamental graded Lie algebras
Let m =
⊕
p<0
gp be a free FGLA of type (n, µ) over C, and let g(m) =
⊕
p∈Z
g(m)p be the prolongation
of m. First of all, we assume that dim g(m) =∞. By Theorem 6.1, g(m) is isomorphic to K(m)
as a GLA, where n = 2m. Since K(m)0 is isomorphic to csp(m,C) and since g(m)0 is isomorphic
to gl(n,C), we see that m = 1. Therefore g(m) is isomorphic to K(1) as a GLA.
Next we assume that dim g(m) < ∞ and g(m)1 6= 0. Since the g(m)0-module g(m)−1 is
irreducible, g(m) is a finite-dimensional simple GLA (see [4, 7]). By Theorem 5.2, g(m) is
isomorphic to one of the following types:
(Bl, {αl}) l = 3, (G2, {α1}).
Thus we get a proof of the following theorem:
10 T. Yatsui
Theorem 7.1. Let m =
⊕
p<0
gp be a free FGLA of type (n, µ) over C, and let g(m) =
⊕
p∈Z
g(m)p
be the prolongation of m. Then one of the following cases occurs:
(a) (n, µ) 6= (n, 2) (n = 2), (2, 3). In this case, g(m)1 = {0}.
(b) (n, µ) = (n, 2) (n = 3), (2, 3). In this case, dim g(m) <∞ and g(m)1 6= {0}. Furthermore
g(m) is isomorphic to a finite-dimensional simple GLA of type (Bn, {αn}) (n = 3) or
(G2, {α1}) (n = 2).
(c) (n, µ) = (2, 2). In this case, dim g(m) = ∞. Furthermore, g(m) is isomorphic to K(1) as
a GLA.
8 Free pseudo-product fundamental graded Lie algebras
An FGLA m =
⊕
p<0
gp equipped with nonzero subspaces e, f of g−1 is called a pseudo-product
FGLA if the following conditions hold:
(i) g−1 = e⊕ f;
(ii) [e, e] = [f, f] = {0}.
The pair (e, f) is called the pseudo-product structure of the pseudo-product FGLA m =
⊕
p<0
gp.
We will also denote by the triplet (m; e, f) the pseudo-product FGLA m =
⊕
p<0
gp with pseudo-
product structure (e, f). Let m =
⊕
p<0
gp (resp. m′ =
⊕
p<0
g′p) be a pseudo-product FGLA with
pseudo-product structure (e, f) (resp. (e′, f′)). We say that two pseudo-product FGLAs (m; e, f)
and (m′; e′, f′) are isomorphic if there exists a GLA isomorphism ϕ of m onto m′ such that
ϕ(e) = e′ and ϕ(f) = f′.
Proposition 8.1. Let m =
⊕
p<0
gp be a pseudo-product FGLA of the µ-th kind with pseudo-
product structure (e, f). If m is a free FGLA of type (n, µ), then n = 2.
Proof. Let (e1, . . . , em) (resp. (f1, . . . , fl)) be a basis of e (resp. f). Since [e, f] = g−2, the
space g−2 is generated by {[ei, fj ] : i = 1, . . . ,m, j = 1, . . . , l} as a vector space, so dim g−2 5 ml.
On the other hand, since m is a free FGLA,
dim g−2 = dim b(g−1, µ)−2 = dim Λ2(g−1) =
(m+ l)(m+ l − 1)
2
,
so ml = (m+l)(m+l−1)
2 . From this fact it follows that m = l = 1. �
Let m =
⊕
p<0
gp be a pseudo-product FGLA of the µ-th kind with pseudo-product structure
(e, f), where µ = 2. m is called a free pseudo-product FGLA of type (m,n, µ) if the following
conditions hold:
(i) dim e = m and dim f = n;
(ii) Let m′ =
⊕
p<0
g′p be a pseudo-product FGLA of the µ-th kind with pseudo-product structure
(e′, f′) and let ϕ be a surjective linear mapping of g−1 onto g′−1 such that ϕ(e) ⊂ e′ and
ϕ(f) ⊂ f′. Then ϕ can be extended uniquely to a GLA epimorphism of m onto m′.
Proposition 8.2. Let m, n and µ be positive integers such that µ = 2.
On Free Pseudo-Product Fundamental Graded Lie Algebras 11
(1) There exists a unique free pseudo-product FGLA of type (m,n, µ) up to isomorphism.
(2) Let m =
⊕
p<0
gp be a free pseudo-product FGLA of type (m,n, µ) with pseudo-product struc-
ture (e, f). We denote by Der(m; e, f)0 the Lie algebra of all the derivations of m preserving
the gradation of m, e and f. Then the mapping Φ : Der(m; e, f)0 3 D 7→ (D|e, D|f) ∈
gl(e)× gl(f) is a Lie algebra isomorphism.
Proof. (1) The uniqueness of a free pseudo-product FGLA of type (m,n, µ) follows from the
definition. Let V be an (m + n)-dimensional vector space and let e, f be subspaces of V such
that V = e ⊕ f, dim e = m and dim f = n. Let a =
⊕
p<0
ap be the graded ideal of b(V, µ)
generated by [e, e] + [f, f]. We set m = b(V, µ)/a, gp = b(V, µ)p/ap. Clearly m =
⊕
p<0
gp is
a pseudo-product FGLA. We show that the factor algebra m is a free pseudo-product FGLA
of type (m,n, µ). First we prove that m is of the µ-th kind. Let n =
⊕
p<0
g′′p be a free FGLA
of type (2, µ) and let e′′ and f′′ be one-dimensional subspaces of g′′−1 such that g′′−1 = e′′ ⊕ f′′.
Let ϕ1 be an injective linear mapping of g′′−1 into V such that ϕ1(e′′) ⊂ e and ϕ1(f′′) ⊂ f.
Let ϕ2 be a linear mapping of V into g′′−1 such that ϕ2 ◦ ϕ1 = 1g′′−1
, ϕ2(e) = e′′ and ϕ2(f) = f′′.
There exists a homomorphism L(ϕ1) (resp. L(ϕ2)) of n (resp. b(V, µ)) into b(V, µ) (resp. n)
such that L(ϕ1)|g′′−1 = ϕ1 (resp. L(ϕ2)|V = ϕ2). Since L(ϕ2)([e, e] + [f, f]) = {0}, L(ϕ2) induces
a homomorphism L̂(ϕ2) of m into n such that L(ϕ2) = L̂(ϕ2)◦π, where π is the natural projection
of b(V, µ) onto m. Since
1n = L(ϕ2) ◦ L(ϕ1) = L̂(ϕ2) ◦ π ◦ L(ϕ1),
π ◦ L(ϕ1) is a monomorphism of n into m, so g−µ 6= {0}. Thus m is of the µ-th kind. Let
m′ =
⊕
p<0
g′p be a pseudo-product FGLA of the µ-th kind with pseudo-product structure (e′, f′)
and let φ be a surjective linear mapping of b(V, µ)−1 onto g′−1 such that φ(e) ⊂ e′ and φ(f) ⊂ f′.
By the definition of a free FGLA, there exists a GLA epimorphism L(φ) of b(V, µ) onto m′
such that L(φ)|b(V, µ)−1 = φ. Since L(φ)([e, e] + [f, f]) ⊂ [e′, e′] + [f′, f′] = {0}, we see that
L(φ)(a) = {0}, so the epimorphism L(φ) induces a GLA epimorphism L̂(φ) of m onto m′ such
that L̂(φ)|g−1 = φ.
(2) We may prove the fact that the mapping Φ is surjective. Let φ be an endomorphism
of g−1 such that φ(e) ⊂ e and φ(f) ⊂ f. By Proposition 2.1 (2), there exists a D ∈ Der(b(V, µ))0
such that D|b(V, µ)−1 = φ. Since D([e, e] + [f, f]) ⊂ [e, e] + [f, f], D induces a derivation D̂ of m
such that D̂|g−1 = φ. �
Remark 8.1. Let m, n, m′, n′ and µ be positive integers with µ = 2, and let m =
⊕
p<0
gp (resp.
m′ =
⊕
p<0
g′p) be a free pseudo-product FGLA of type (m,n, µ) (resp. (m′, n′, µ)) with pseudo-
product structure (e, f) (resp. (e′, f′)). Furthermore let ϕ be a linear mapping of g−1 into g′−1
such that ϕ(e) ⊂ e′ and ϕ(f) ⊂ f′.
(1) From the proof of Proposition 8.2, there exists a unique GLA homomorphism L̂(ϕ) of m
into m′ such that L̂(ϕ)|g−1 = ϕ. If ϕ is injective, then L̂(ϕ) is a monomorphism.
(2) Assume that m = n = 1 and ϕ is injective. Then L̂(ϕ)(m) is a graded subalgebra of m′
isomorphic to a free FGLA of type (2, µ). From this result, the subalgebra of m′ generated
by a nonzero element X of e′ and a nonzero element Y of f′ is a free FGLA of type (2, µ).
Let m =
⊕
p<0
gp be a pseudo-product FGLA of the µ-th kind with pseudo-product struc-
ture (e, f). We denote by g0 the Lie algebra of all the derivations of m preserving the gradation
12 T. Yatsui
of m, e and f:
g0 = {D ∈ Der(g)0 : D(e) ⊂ e, D(f) ⊂ f}.
The prolongation g =
⊕
p∈Z
gp of (m, g0) is called the prolongation of (m; e, f).
A transitive GLA g =
⊕
p∈Z
gp is called a pseudo-product GLA if there are given nonzero
subspaces e and f of g−1 satisfying the following conditions:
(i) The negative part g− is a pseudo-product FGLA with pseudo-product structure (e, f);
(ii) [g0, e] ⊂ e and [g0, f] ⊂ f.
The pair (e, f) is called the pseudo-product structure of the pseudo-product GLA g =
⊕
p∈Z
gp. If
the g0-modules e and f are irreducible, then the pseudo-product GLA g =
⊕
p∈Z
gp is said to be of
irreducible type.
The following lemma is due to N. Tanaka (cf. [9]). Here we give a proof for the convenience
of the readers.
Lemma 8.1. Let g =
⊕
p∈Z
gp be a pseudo-product GLA of depth µ with pseudo-product struc-
ture (e, f).
(1) If g− is non-degenerate, then g is finite-dimensional.
(2) If g =
⊕
p∈Z
gp is of irreducible type and µ = 2, then g is finite-dimensional.
Proof. (1) The proof is essentially due to the proof of [11, Corollary 3 to Theorem 11.1]. For
p ∈ Z, we set hp = {X ∈ gp : [X, g5−2] = {0}}. We define I ∈ gl(g−1) as follows: I(x) = −
√
−1x
for x ∈ e, I(x) =
√
−1x for x ∈ f. Then I2 = −1, I([a, x]) = [a, I(x)] and [I(x), I(y)] = [x, y]
for a ∈ g0 and x, y ∈ g−1. We put 〈x, y〉 = [I(x), y] for x, y ∈ g−1. Then 〈x, y〉 = 〈y, x〉, and for
x ∈ g−1, 〈x, g−1〉 = {0} implies x = 0, since g− is non-degenerate. Also 〈[a, x], y〉+ 〈x, [a, y]〉 = 0
and [[b, x], y] = [[b, y], x] for a ∈ h0, b ∈ h1 and x, y ∈ g−1. Then, for b ∈ h1, x, y, z ∈
g−1, we have 〈[[b, x], y], z〉 = −〈y, [[b, x], z]〉 = −〈y, [[b, z], x]〉 = 〈[[b, z], y], x〉 = 〈[[b, y], z], x〉 =
−〈z, [[b, y], x]〉 = −〈[[b, x], y], z〉, so 〈[[b, x], y], z〉 = 0. By transitivity of g, h1 = {0}. Therefore
by [11, Corollary 1 to Theorem 11.1], g is finite-dimensional.
(2) We may assume that g1 6= {0}. By [16, Lemma 2.4], the g0-modules e, f are not isomorphic
to each other. We put d = {X ∈ g−1 : [X, g−1] = {0}}; then d is a g0-submodule of g−1. Hence
d = {0}, d = e, d = f or d = g−1. If d 6= {0}, then g−2 = [e, f] = {0}, which is a contradiction.
Thus g− is non-degenerate. By (1), g is finite-dimensional. �
The prolongation of a pseudo-product FGLA becomes a pseudo-product GLA. By Propo-
sition 8.2 (2), the prolongation of a free pseudo-product FGLA is a pseudo-product GLA of
irreducible type. By Lemma 8.1 (2), the prolongation of a free pseudo-product FGLA is finite-
dimensional.
Proposition 8.3. Let m =
⊕
p<0
gp be a free pseudo-product FGLA of type (m,n, µ) with pseudo-
product structure (e, f) and let g =
⊕
p∈Z
gp be the prolongation of (m; e, f).
(1) g0 is isomorphic to gl(e)⊕ gl(f) as a Lie algebra.
(2) g−2 is isomorphic to e⊗ f as a g0-module. In particular, dim g−2 = mn.
On Free Pseudo-Product Fundamental Graded Lie Algebras 13
(3) g−3 is isomorphic to S2(e) ⊗ f ⊕ S2(f) ⊗ e as a g0-module. In particular, dim g−3 =
mn(m+n+2)
2 .
Proof. (1) This follows from Proposition 8.2 (2).
(2) Let a =
⊕
p<0
ap be the graded ideal of b(g−1, µ) generated by [e, e]+[f, f]. By the construction
of b(g−1, µ)−2, a−2 is isomorphic to Λ2(e) ⊕ Λ2(f), so g−2 = b(g−1, µ)−2/a−2 is isomorphic to
e⊗ f.
(3) By the construction of b(g−1, µ)−3, b(g−1, µ)−3 is isomorphic to
(e⊕ f)⊗ Λ2(e⊕ f)/Λ3(e⊕ f) ∼= (e⊗ e⊗ f)⊕ (e⊗ f⊗ f).
Moreover, a−3 is isomorphic to
(e⊕ f)⊗ Λ2(e)⊕ (e⊕ f)⊗ Λ2(f)/Λ3(e⊕ f) ∼= e⊗ Λ2(e)⊕ f⊗ Λ2(f).
Hence g−3 = b(g−1, µ)−3/a−3 is isomorphic to
(e⊗ e⊗ f)/Λ2(e)⊗ f⊕ (e⊗ f⊗ f)/e⊗ Λ2(f) ∼= S2(e)⊗ f⊕ S2(f)⊗ e.
This completes the proof. �
Proposition 8.4. Let m =
⊕
p<0
gp be a pseudo-product FGLA of the µ-th kind with pseudo-
product structure (e, f), where µ = 2. We denote by c the centralizer of g−2 in g−1. Let g =
⊕
p∈Z
gp
be the prolongation of (m; e, f). Assume that g0 is isomorphic to gl(e)⊕ gl(f) as a Lie algebra.
(1) If µ = 2, then m =
⊕
p<0
gp be a free pseudo-product FGLA.
(2) If µ = 3 and c 6= {0}, then (m; e, f) is not a free pseudo-product FGLA.
(3) If µ = 3 and c = {0}, then (m; e, f) is a free pseudo-product FGLA.
Proof. Let m̌ =
⊕
p<0
ǧp be the free pseudo-product FGLA of type (m,n, µ) with pseudo-product
structure (ě, f̌) such that ǧ−1 = g−1, ě = e and f̌ = f. Let ǧ =
⊕
p∈Z
ǧp be the prolongation of
(m̌; ě, f̌). There exists a GLA epimorphism ϕ of m̌ onto m such that the restriction ϕ|ǧ−1 is the
identity mapping. Since the mapping ǧ0 3 D 7→ (D|e, D|f) ∈ gl(e) × gl(f) is an isomorphism,
ϕ can be extended to be a homomorphism ϕ̌ of
⊕
p50
ǧp onto
⊕
p50
gp. Let a be the kernel of ϕ̌;
then a is a graded ideal of
⊕
p50
ǧp. We set ap = a ∩ ǧp; then a =
⊕
p50
ap. Since the restriction of
ϕ̌ to ǧ−1 ⊕ ǧ0 is injective, ap = {0} for p = −1. Also each ap is a ǧ0-submodule of ǧp. Since
the ǧ0-module ǧ−2 is irreducible (Proposition 8.3 (2)), ϕ|g−2 is injective. If µ = 2, then ϕ is an
isomorphism. This proves the assertion (1). Now we assume that µ = 3. Then
ǧ−3 = [[e, f], f]⊕ [[e, f], e].
Since ǧ0-modules [[e, f], f] and [[e, f], e] are irreducible and not isomorphic to each other (Propo-
sition 8.3 (3)), one of the following cases occurs: (i) a−3 = [[e, f], f]; (ii) a−3 = [[e, f], e];
(iii) a−3 = {0}. If a−3 = [[e, f], f] (resp. a−3 = [[e, f], e]), then c = f (resp. c = e). Also
since g0-modules e, f are irreducible and not isomorphic to each other, one of the following cases
occurs: (i) c = e; (ii) c = f; (iii) c = {0}. If c = e (resp. c = f), then a−3 = [[e, f], e] (resp.
a−3 = [[e, f], f]). In this case, ϕ is not injective. Hence (m; e, f) is not free. If c = {0}, then
a−3 = {0}. Hence ϕ|ǧ−3 is an isomorphism. In particular, if µ = 3, then (m; e, f) is free. �
14 T. Yatsui
Example 8.1. Let V and W be finite-dimensional vector spaces and k = 1. We set
Ck(V,W ) =
−1⊕
p=−k−1
Ck(V,W )p,
Ck(V,W )p = W ⊗ Sk+p+1(V ∗), −k − 1 5 p 5 −2,
Ck(V,W )−1 = V ⊕ (W ⊗ Sk(V ∗)).
The bracket operation of Ck(V,W ) is defined as follows:
[W,V ] = {0}, [V, V ] = {0}, [W ⊗ Sr(V ∗),W ⊗ Ss(V ∗)] = {0},
[w ⊗ sr, v] = w ⊗ (vy sr) for v ∈ V, w ∈W, sr ∈ Sr(V ∗).
Equipped with this bracket operation, Ck(V,W ) becomes a pseudo-product FGLA of the (k+1)-
th kind with pseudo-product structure (V,W ⊗ Sk(V ∗)), which is called the contact algebra of
order k of bidegree (n,m), where n = dimV and m = dimW (cf. [14, p. 133]). We assume that
Ck(V,W ) is a free pseudo-product FGLA. Since
dimCk(V,W )−2 = m
(
n+ k − 2
k − 1
)
, dimV dim(W ⊗ Sk(V ∗)) = nm
(
n+ k − 1
k
)
,
we get n = 1. Since W ⊗Sk(V ∗) is contained in the centralizer of Ck(V,W )−2 in Ck(V,W )−1, we
get k = 1. Thus we obtain that Ck(V,W ) is a free pseudo-product FGLA if and only if k = 1,
n = 1.
Example 8.2. Let g =
⊕
p∈Z
gp be a finite-dimensional simple GLA of type (Am+n, {αm, αm+1}).
We set e = g
(m)
−1 , f = g
(m+1)
−1 . Then (g−; e, f) is a pseudo-product FGLA. Since dim e = m,
dim f = n and dim g−2 = mn, the pseudo-product FGLA (g−; e, f) is a free pseudo-product
FGLA of type (m,n, 2) (Proposition 8.3 (2)). Also g =
⊕
p∈Z
gp is the prolongation of g− except
for the following cases (see [15]):
(1) m = n = 1. In this case, the prolongation of g− is isomorphic to K(1).
(2) m = 1 or n = 1 and l = max{m,n} = 2. In this case, the prolongation of g− is isomorphic
to W (l + 1; s), where s = (1, 2, . . . , 2).
Example 8.3. Let V and W be finite-dimensional vector spaces such that dimV = m = 1 and
dimW = n = 1. We set
g−1 = V ⊕W, g−2 = V ⊗W,
g−3 = V ⊗ S2(W )⊕ S2(V )⊗W, m = g−1 ⊕ g−2 ⊕ g−3.
The bracket operation of m is defined as follows:
[g−3, g−1 ⊕ g−2] = [g−2, g−2] = {0}, [V, V ] = [W,W ] = {0},
[v, w] = −[w, v] = v ⊗ w, [v, v′ ⊗ w] = −[v′ ⊗ w, v] = v } v′ ⊗ w,
[v ⊗ w,w′] = −[w′, v ⊗ w] = v ⊗ w } w′,
where v, v′ ∈ V and w,w′ ∈W . Equipped with this bracket operation, m becomes a free pseudo-
product FGLA of type (m,n, 3) with pseudo-product structure (V,W ) (Proposition 8.3).
On Free Pseudo-Product Fundamental Graded Lie Algebras 15
Theorem 8.1. Let m =
⊕
p<0
gp be a free pseudo-product FGLA of type (m,n, µ) with pseudo-
product structure (e, f) over C. Furthermore let g =
⊕
p∈Z
gp (resp. g(m) =
⊕
p∈Z
g(m)p) be the
prolongation of (m; e, f) (resp. m).
(1) Assume that dim g(m) = ∞. Then m = 1 or n = 1, and µ = 2. Furthermore g =⊕
p∈Z
gp is isomorphic to a finite-dimensional simple GLA of type (Al+1, {α1, α2}), where
l = max{m,n}. If l = 1, then g(m) is isomorphic to K(1). If l = 2, then g(m) is
isomorphic to W (l + 1; s), where s = (1, 2, . . . , 2).
(2) If g1 6={0}, then g =
⊕
p∈Z
gp is a finite-dimensional simple GLA of type (Am+n, {αm, αm+1}).
Proof. (1) For p=−1, we put hp = {X ∈ g(m)p : [X, g5−2] = {0}}. Assume that dim g(m) =∞
and µ = 3. By Proposition 8.4 (2), h−1 = {0} . Since [h0, g−1] ⊂ h−1 = {0}, we see that h0 = {0}.
By [11, Corollary 1 to Theorem 11.1], we obtain that dim g(m) < ∞, which is a contradiction.
Thus we see that µ = 2 if dim g(m) =∞. The remaining assertion follows from Example 8.2.
(2) Assume that g1 6= {0} and µ = 3. By transitivity of g, [g1, e] 6= {0} or [g1, f] 6= {0}.
We may assume that [g1, e] 6= {0}. Then there exists an irreducible component g′1 of the g0-
module g1 such that [g′1, e] 6= {0} and [g′1, f] = {0}. The subalgebra e ⊕ [e, g′1] ⊕ g′1 is a simple
GLA of the first kind. Since g0 is isomorphic to gl(e)⊕gl(f), e⊕ [e, g′1]⊕g′1 is of type (Am, {α1}).
Let D be a nonzero element of g′1. There exist λ ∈ e∗ and η ∈ f∗ such that
[[D,Z], U ] = λ(U)Z + λ(Z)U, [[D,Z],W ] = η(Z)W,
where Z,U ∈ e and W ∈ f (cf. [12, p. 4]). Let X (resp. Y ) be a nonzero element of e (resp. f).
Since the subalgebra generated by X,Y is a free FGLA of type (2, µ) (Remark 8.1 (2)),
ad(X)µ(Y ) = 0, ad(X)µ−1(Y ) 6= 0,
ad(Y ) ad(X)µ−1(Y ) = 0, ad(Y ) ad(X)µ−2(Y ) 6= 0
(Lemma 2.1). By induction on µ, we see that
0 = ad(D) ad(X)µ(Y ) = (µ(µ− 1)λ(X) + µη(X)) ad(X)µ−1(Y ),
0 = ad(D) ad(Y ) ad(X)µ−1(Y )
= ((µ− 1)(µ− 2)λ(X) + (µ− 1)η(X)) ad(Y ) ad(X)µ−2(Y ).
Since
det
[
µ(µ− 1) µ
(µ− 1)(µ− 2) µ− 1
]
= µ(µ− 1) 6= 0,
we see that λ(X) = η(X) = 0, which is a contradiction. Thus we obtain that µ = 2 if
dim g1 6= {0}. From Example 8.2, it follows that g =
⊕
p∈Z
gp is a simple GLA of type (Am+n,
{αm, αm+1}) if dim g1 6= {0}. �
9 Automorphism groups of the prolongations
of free pseudo-product fundamental graded Lie algebras
For a GLA g =
⊕
p∈Z
gp we denote by Aut(g)0 the group of all the automorphisms of g preserving
the gradation of g:
Aut(g)0 = {ϕ ∈ Aut(g) : ϕ(gp) = gp for all p ∈ Z}.
16 T. Yatsui
Proposition 9.1. Let m =
⊕
p<0
gp be an FGLA and let g(m) =
⊕
p∈Z
g(m)p be the prolongation
of m. The mapping Φ : Aut(g(m))0 3 φ 7→ φ|m ∈ Aut(m)0 is an isomorphism.
Proof. It is clear that Φ is a group homomorphism. We prove that Φ is injective. Let φ be an
element of Ker Φ. Assume that φ(X) = X for all X ∈ g(m)p (p < k). For X ∈ g(m)k, Y ∈ g−1,
[φ(X)−X,Y ] = φ([X,Y ])− [X,Y ].
Since [X,Y ] ∈ g(m)k−1, we have [φ(X)−X,Y ] = 0. By transitivity, φ(X) = X. By induction,
we have proved φ to be the identity mapping. Hence Φ is a monomorphism.
We prove that Φ is surjective. Let ϕ ∈ Aut(m)0. We construct the mapping ϕp : g(m)p →
g(m)p inductively as follows: First for X ∈ g(m)0, we set ϕ0(X) = ϕXϕ−1. Then for Y,Z ∈ m
ϕ0(X)([Y,Z]) = [ϕ(X(ϕ−1(Y ))), Z] + [Y, ϕ(X(ϕ−1(Z)))],
so ϕ0(X) ∈ g(m)0. Furthermore we can prove easily that [ϕ0(X), ϕp(Y )] = ϕp([X,Y ]) for X ∈ g0
and Y ∈ gp (p 5 0). Here for p < 0 we set ϕp = ϕ|g(m)p. Assume that we have defined linear
isomorphisms ϕp of g(m)p onto itself (0 5 p < k) such that
ϕr+s([X,Y ]) = [ϕr(X), ϕs(Y )]
for X ∈ g(m)r, Y ∈ g(m)s (r + s < k, r < k, s < k). For X ∈ g(m)k we define ϕk(X) ∈
Hom(m,
⊕
p5k−1
g(m)p)k as follows:
ϕk(X)(Y ) = ϕk+s([X,ϕ
−1(Y )]), Y ∈ gs, s < 0.
For Y ∈ gs, Z ∈ gt (s, t < 0),
ϕk(X)([Y, Z]) = ϕk+t+s([X,ϕ
−1([Y, Z]])
= ϕk+s+t([[X,ϕ
−1(Y )], ϕ−1(Z)] + [ϕ−1(Y ), [X,ϕ−1(Z)]])
= [ϕk+s([X,ϕ
−1(Y )]), Z] + [Y, ϕk+t([X,ϕ
−1(Z)])]
= [ϕk(X)(Y ), Z] + [Y, ϕk(X)(Z)],
so ϕk(X) ∈ g(m)k. Next we prove that for X ∈ gp, Y ∈ gq (p+ q = k, 0 5 p 5 k, 0 5 q 5 k),
ϕk([X,Y ]) = [ϕp(X), ϕq(Y )].
For Z ∈ gs (s < 0),
[[ϕp(X), ϕq(Y )], Z] = [ϕp(X), [ϕq(Y ), Z]]− [ϕq(Y ), [ϕp(X), Z]]
= ϕp+q+s([X, [Y, ϕ
−1(Z)]]− [Y, [X,ϕ−1(Z)]])
= ϕp+q+s([[X,Y ], ϕ−1(Z)]) = [ϕk([X,Y ]), Z].
By transitivity, we see that ϕk([X,Y ]) = [ϕp(X), ϕq(Y )]. We define a mapping ϕ̌ of g(m) into
itself as follows:
ϕ̌(X) =
{
ϕ(X), X ∈ m,
ϕk(X), k = 0, X ∈ g(m)k.
From the above results and the definition of ϕk (k = 0), we see that ϕ̌ is a GLA homomorphism.
Assume that ϕk−1 (k = 0) is a linear isomorphism. For X ∈ g(m)k, if ϕk(X) = 0, then
0 = [ϕk(X), Y ] = ϕk−1([X,ϕ−1(Y )]) for all Y ∈ g−1. By transitivity, we see that X = 0, so ϕk
is a linear isomorphism. Therefore ϕ̌ is an automorphism of g(m). �
On Free Pseudo-Product Fundamental Graded Lie Algebras 17
Theorem 9.1. Let m =
⊕
p<0
gp be a free FGLA over C, and let g(m) =
⊕
p∈Z
g(m)p be the prolon-
gation of m. The mapping Φ : Aut(g(m))0 3 φ 7→ φ|g−1 ∈ GL(g−1) is an isomorphism.
Proof. We may assume that m is a universal FGLA b(g−1, µ) of the µ-th kind. By Corollary 1
to Proposition 3.2 of [11], the mapping Aut(m)0 3 a 7→ a|g−1 ∈ GL(g−1) is an isomorphism.
By Proposition 9.1, we see that the mapping Φ : Aut(g(m))0 3 φ 7→ φ|g−1 ∈ GL(g−1) is an
isomorphism. �
For a pseudo-product GLA g =
⊕
p∈Z
gp with pseudo-product structure (e, f), we denote by
Aut(g; e, f)0 the group of all the automorphisms of g preserving the gradation of g, e and f:
Aut(g; e, f)0 = {ϕ ∈ Aut(g)0 : ϕ(e) = e, ϕ(f) = f}.
Theorem 9.2. Let m =
⊕
p<0
gp be a free pseudo-product FGLA of type (m,n, µ) with pseudo-
product structure (e, f) over C, and let g =
⊕
p∈Z
gp be the prolongation of (m; e, f). The mapping
Φ : Aut(g; e, f)0 3 φ 7→ (φ|e, φ|f) ∈ GL(e) × GL(f) is an isomorphism. Furthermore if dim e 6=
dim f, then Aut(g; e, f)0 = Aut(g)0.
Proof. Clearly Φ is a monomorphism. We show that Φ is surjective. Let (φ1, φ2) be an element
of GL(e)×GL(f). We set φ = φ1⊕φ2 ∈ GL(g−1). By Corollary 1 to Proposition 3.2 of [11], there
exists an element ϕ1 ∈ Aut(b(g−1, µ))0 such that ϕ1|g−1 = φ. Since ϕ1([e, e]+[f, f]) = [e, e]+[f, f],
ϕ1 induces an element ϕ2 ∈ Aut(m; e, f)0 such that ϕ2|g−1 = φ. By Proposition 9.1, there exists
ϕ3 ∈ Aut(g(m))0 such that ϕ3|m = ϕ2. We prove that ϕ3(g) = g. For X0 ∈ g0 and Y ∈ e,
we see that [ϕ3(X0), Y ] = ϕ3([X0, ϕ
−1
3 (Y )]) ∈ ϕ3(e) = e, so ϕ3(X0)(e) ⊂ e. Similarly we get
ϕ3(X0)(f) ⊂ f. Thus we obtain that ϕ3(g0) = g0. Now we assume that ϕi(gi) = gi for 0 5 i 5 k.
Then for Xk+1 ∈ gk+1 and Y ∈ gp (p < 0), we see that [ϕ3(Xk+1), Y ] = ϕ3([Xk+1, ϕ
−1
3 (Y )]) ∈
ϕ3(gp+k+1) = gp+k+1, so ϕ3(gk+1) ⊂ gk+1. Hence ϕ3(g) = g and Φ is surjective. Now we assume
that dim e 6= dim f. Let ϕ ∈ Aut(g)0. Since g0-modules e and f are not isomorphic to each
other, we see that (i) ϕ(e) = e, ϕ(f) = f or (ii) ϕ(e) = f, ϕ(f) = e. According to the assumption
dim e 6= dim f, we get ϕ(e) = e, ϕ(f) = f, so ϕ ∈ Aut(g; e, f)0. �
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http://dx.doi.org/10.1007/s10711-007-9205-1
1 Introduction
2 Free fundamental graded Lie algebras
3 Universal fundamental graded Lie algebras
4 The prolongations of fundamental graded Lie algebras
5 Finite-dimensional simple graded Lie algebras
6 Graded Lie algebras W(n), K(n) of Cartan type
7 Classification of the prolongations of free fundamental graded Lie algebras
8 Free pseudo-product fundamental graded Lie algebras
9 Automorphism groups of the prolongations of free pseudo-product fundamental graded Lie algebras
References
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