1. Introduction and
Preliminaries
The study of real algebras without divisors of zero started since the
discovery of (Hamilton’s
quaternions) and
(Cayley’s octonions).
Frobenius proves that , and are the only associative,
quadratic real algebras without divisors of zero [1,2]. Zorn shows that every alternative,
quadratic, non associative real algebra without divisors of zero is
isomorphic to the Cayley algebra [1-3]. So, every alternative, quadratic real
algebra without divisors of zero is isomorphic to either , , , or (Frobenius-Zorn theorem).
Many researchers have given minimum conditions for an algebra to be
isomorphic to , , , or . The Frobenius-Zorn theorem
will then be either improved or extended.
By definition an absolute valued real algebra is an algebra endowed with an absolute
value, i.e. a norm on
the vector space of
satisfying for
all . Clearly
absolute valued algebras have no nonzero divisor of zero, and hence have
no nonzero joint divisor of zero. Albert shows that , , and are the only absolute valued
real algebraic algebras with unit [4, Theorem
2].
An algebra is called
division algebra, if and are bijective for all . Cuenca proves that , , and are the only division normed
real algebras satisfying the middle Moufang identity [5, Theorem 2.3] and he also classified the
division normed real algebras satisfying the left or right Moufang
identity [5,Theorems 2.1 and 2.2]. In [6], Cuenca also studied the
composition algebras satisfying Moufang identities.
Let be a normed
algebra. Recall that an element
of is said to be a
joint topological divisor of zero in if there is a sequence of norm-one elements of satisfying and . If is a weakly alternative
normed real unital algebra with no nonzero joint topological divisor of
zero, then is
isomorphic to , , or [7, Theorem III.5.13].
The algebras , , and are the only normed
alternative real algebras with no nonzero joint topological divisor of
zero [8, Theorem 2.5.50].
In [8, Theorem 2.5.29], Cabrera and
Rodriguez prove also the following result.
Theorem 1. Let be a nonzero alternative real
algebraic algebra with no nonzero joint divisor of zero. Then is isomorphic to , , , or .
In this paper, we prove that the Frobenius-Zorn theorem remains if we
replace quadratic by algebraic and alternative by weakly alternative,
middle Moufang identity, or left and right Moufang identities.
In this last part of this paragraph, we are going to discuss the
assumptions on in the
main theorems.
We recall that if is
alternative, then
satisfies the three Moufang identity and is weakly alternative. We
precise also that every quadratic algebra is algebraic.
Let . We recall that is obtained
by endowing the normed space with the product ,
where means
the standard involution. We note that these algebras are algebraic, but
are not quadratic.
We recall that
and are obtained
by endowing the normed space with the products , and , respectively,
where means
the standard involution. These algebras are algebraic and it is easy to
verify that
satisfies right Moufang identity and verifies the left
Moufang identity. However, none of the algebras and is alternative. This
implies that neither of the two identities of Moufang of the assertion 2
from Theorem 3 can be removed.
2. Notations and Preliminary
In this paper, is a nonzero
algebra over a field of
characteristic zero. We recall that , , denote respectively
the associator, the commutator, and the subalgebra of generated by .
An element of is said to be a divisor of
zero in if there exists
such that
or , and that is said to be a joint divisor of
zero in if there is such that and .
An element of an algebra is
said to be isotropic whenever . We note that isotropic elements are nonzero joint
divisors of zero.
Let be an algebra over . The centre of is defined as the subset of consisting of those elements such that and is
denoted by . Elements of are called central elements of . We note that becomes an associative and
commutative subalgebra of .
An algebra is called
alternative (resp. flexible) if and
(resp. ) for all .
An algebra is called
power-associative (resp. algebraic) if is associative (resp. is finite dimensional) for all .
We also remember that an algebra satisfies:
the middle Moufang identity, if , for all ,
the right Moufang identity, if , for all ,
the left Moufang identity, if , for all .
An algebra is called
non-commutative Jordan, if is
flexible and satisfies for all . A non
commutative Jordan -algebra is said to be weakly alternative if it
satisfies the identity for all [7].
An algebra is called quadratic
algebra, if contains a unit
element and are linearly dependent
for all .
Let be a quadratic
algebra over . By
definition, for each , there
are and of such that If belongs to , the
scalars and in (1) are
uniquely determined. Otherwise, we have for a unique , and we set and , so that (1) is fulfilled. The mappings and , from to , defined in this way are
called the trace function and the algebraic norm function on , respectively.
It follows from [8,Proposition 2.5.12] that
, where
. So, for every , we have unique writing
for and . We recall that is called the scalar part of
, and is called the vector part of . If for , we denote by and the
scalar and vector parts of ,
respectively, that is to say, if we write then we are provided with a bilinear form
and a bilinear mapping . We note that, for , we have (by (2.5.9) in
[8]) and also , so that,
since lies in , we get . Therefore is an anticommutative
algebra.
Every right alternative algebra is power-associative [9, Lemma 1]. The Mikheev’s theorem proves
that if is right alternative
algebra, then
[10]. By switching to the
opposite algebra, we deduce the following corollary:
Corollary 1. Let be an algebra over without isotropic elements.
Then the following assertions are equivalent:
is a right alternative
algebra,
is a left alternative
algebra,
is an alternative
algebra.
3. Main Results
Proposition 1. Let be a nonzero algebraic real algebra
with no nonzero joint divisor of zero, and with nonzero centre. If satisfies one of the Moufang
identities, then is a quadratic
alternative algebra.
Proof. Since the centre of is nonzero, it follows from [8, Proposition 2.5.33] that is unital.
We now study each one of the Moufang identities separately.
is a middle Moufang
algebra.
Taking in the middle
Moufang identity
we get , that is, is a flexible algebra, and
consequently, taking we obtain
that . Moreover, taking
in the middle Moufang
identity we see that . Since it
follows that ,
that is, . Thus is a power-associative algebra [11], and hence, by [8,Proposition 2.5.10(ii)], is quadratic.
Let us write . Keeping in mind that is flexible, by [8,Proposition 2.5.18(ii)] we know that For any , we have that and hence where in the last equality we have used the
symmetry of , and the
anticommutativity of . On the
other hand, where in
the second equality we have used the anticommutativity of . Therefore, where in the last equality we have used the
anticommutativity of .
Since (by
the middle Moufang identity), it follows from (3) and (4) that Therefore, by [8, Proposition 2.5.18(iii)], the algebraic norm fonction on admits composition. Moreover is nondegenerate because of [8, Lemma 2.5.15 and Proposition 2.5.18(i)]. Finally, by [8, Corollary 2.5.19(ii)], we conclude that is alternative.
Case (r) is a right Moufang
algebra.
Taking in the right
Moufang identity we get , that is, is a right alternative algebra.
Therefore, by Corollary 1, is
alternative. Moreover, by [8, Proposition
2.5.10(ii)], we conclude that is quadratic.
Case (1) is a left Moufang algebra.
This case follows from Case (r) by passing to the opposite
algebra. 
Combining the above proposition with Theorem 1 we obtain the
following.
Theorem 2. Let be a nonzero algebraic real algebra
with no nonzero joint divisor of zero and with nonzero centre. If satisfies one of the Moufang
identities, then is isomorphic to
, , , or .
The following result provides us with sufficient conditions for an
algebra satisfying one of the Moufang identities to have a nonzero
centre.
Proposition 2. Let be an algebra over with no nonzero divisor of
zero. We have:
If satisfies the
middle or right Moufang identity, and if has a right unit, then is unital.
If satisfies the
middle or left Moufang identity, and if has a left unit, then is unital.
Proof. (i) Suppose that has a right unit .
If satisfies the middle
Moufang identity, then, taking in that identity, we get , hence , so , and as a result is the unit of .
If satisfies the right Moufang
identity, then, taking in
that identity, we get ,
hence , so , and as a result is the unit of .
The proof of (ii) is similar. 
Proposition 2 yields the next consequence of Theorem 2.
Corollary 2. Let be a nonzero algebraic real algebra
with no nonzero divisor of zero. Suppose that satisfies one of the following
conditions:
is a middle or right
Moufang algebra, and has a right
unit.
is a middle or left
Moufang algebra, and has a left
unit.
Then is isomorphic to
, , , or .
Theorem 3. Let be a nonzero algebraic real algebra
with no nonzero divisor of zero. Then the following assertions are
equivalent:
is a middle Moufang
algebra,
is a right and left
Moufang algebra,
is
alternative,
is isomorphic to , , , or .
Proof. The implications , , and are clear, whereas
the implication follows from Theorem 1.
. Let . Then the subalgebra
is a finite-dimensional
division real algebra. Since every finite-dimensional real algebra can
be provided with an algebra norm [8,Proposition 1.1.7], it follows from [5,Theorem 2.3] that
is isomorphic to , , , or , hence is power-associative, and so is associative. Now, it follows from
the arbitrariness of in that is power-associative. Therefore, by
[8,Proposition 2.5.10(ii)], is quadratic, and hence has a unit. Finally, is alternative because of Proposition
1.
. Argue as
in the above paragraph, with [5,Theorems 2.1 and 2.2] instead of [5,Theorems 2.3]. 
Proposition 3. Let be a nonzero flexible quadratic algebra
over with no nonzero
joint divisor of zero. If
satisfies the identity , then is alternative.
Proof. Let us write .
Since is flexible it follows from
[8,Proposition 2.5.18(ii)] that For any , we have that and hence the
symmetry of and the
anticommutativity of , yield
. Moreover, keeping
in mind the second assertion in (5) and the
anticommutativity of , we see
that Therefore and Hence and consequently For the
sake of comfortability, let us set . Note that and that, keeping in mind (6), also
Therefore and Thus
is a joint divisor of zero of ,
and consequently .
Finally, looking at [8, Proposition 2.5.18(iv)], we
conclude that is
alternative. 
Theorem 4. Let be a nonzero power-associative flexible
algebraic real algebra with no nonzero joint divisor of zero. If satisfies the identity , then is isomorphic to , , , or .
Proof. By [8, Proposition 2.5.10(ii)],
is quadratic. Suppose that satisfies the identity . Then, by Proposition 3, is alternative, and hence is isomorphic to , , , or because of Theorem 1. 
Recall the definition of a weakly alternative algebra, as well as the
fact that non-commutative Jordan algebras are power-associative [8, Proposition 2.4.19], we have the following
corollary.
Corollary 3. Every nonzero weakly alternative
algebraic real algebra with no nonzero joint divisor of zero is
isomorphic to , , or .
Acknowledgments
The authors would like to thank the referee for his careful reading
of this article and his suggestions that improved the final version.
Conflict of
Interest
The authors declare no conflict of interest.