# Freudenthal-Kantor triple system

A triple system considered for constructing all simple Lie algebras (cf. Lie algebra), and introduced as an algebraic system which is a generalization both of the algebraic systems appearing in the metasymplectic geometry developed by H. Freudenthal and of a generalized Jordan triple system of second order developed by I.L. Kantor.

Recall that a triple system is a vector space $V$ over a field $\Phi$ together with a $\Phi$-trilinear mapping $V \times V \times V \rightarrow V$.

For $\epsilon = \pm 1$, a vector space $U ( \varepsilon )$ over a field $\Phi$ with the trilinear product $\langle x y z \rangle$ is called a Freudenthal–Kantor triple system if

$$\tag{a1} \langle a b \langle c d e \rangle \rangle = \langle \langle a b c \rangle \rangle + \varepsilon \langle c \langle b a d \rangle e \rangle + \langle c d \langle a b e \rangle \rangle,$$

$$\tag{a2} K ( L ( a , b ) c , d ) + K ( c , L ( a , b ) d ) + K ( a , K ( c , d ) b ) = 0,$$

where $L ( a , b ) c = \langle a b c \rangle$ and $K ( a , b ) c = \langle a c b \rangle - \langle b c a \rangle$.

In particular, a Freudenthal–Kantor triple system $U ( \varepsilon )$ is said to be balanced if there exists a bilinear form $\langle \, .\, ,\, . \, \rangle$ such that $K ( a , b ) = \langle a , b \rangle \operatorname{Id}$, for all $a,b \in U ( \varepsilon )$.

This balancing property is closely related to metasymplectic geometry.

Note that if $\varepsilon = - 1$ and $K ( a , b ) \equiv 0$ (identically), then the Freudenthal–Kantor triple system reduces to a Jordan triple system.

As the notion of a Freudenthal–Kantor triple system includes the notions of a generalized Jordan triple system of second order, a structurable algebra, and an Allison–Hein triple system, it is useful in obtaining all Lie algebras, without the use of root systems and Cartan matrices.

Let $V$ be a vector space with a bilinear form $\langle x , y \rangle = - \varepsilon \langle y , x \rangle$. Then $V$ is a Freudenthal–Kantor triple system with respect to the triple product $\langle x y z \rangle : = \langle y , z \rangle x$. In particular, it is important that the linear span $\mathbf{k} : = \{ K ( a , b ) \} _ { \text{span} }$ of the set $K ( a , b )$ makes a Jordan triple system of $( \text { End } U ( \varepsilon ) ) ^ { + }$ with respect to the triple product $\{ A B C \} : = 1 / 2 ( A B C + C B A )$.

Let $U ( \varepsilon )$ be a Freudenthal–Kantor triple system. The vector space $U ( \varepsilon ) \oplus U ( \varepsilon )$ becomes a Lie triple system with respect to the triple product defined by

\begin{equation*} \left[ \left( \begin{array} { l } { a } \\ { b } \end{array} \right) \left( \begin{array} { l } { c } \\ { d } \end{array} \right) \left( \begin{array} { l } { e } \\ { f } \end{array} \right) \right] : = \end{equation*}

\begin{equation*} := \left( \begin{array} { c c } { L ( a , d ) - L ( c , b ) } & { K ( a , c ) } \\ { - \varepsilon K ( b , d ) } & { \varepsilon ( L ( d , a ) - L ( b , c ) ) } \end{array} \right) \left( \begin{array} { l } { e } \\ { f } \end{array} \right). \end{equation*}

Using this, one can obtain the Lie triple system $U ( \varepsilon ) \oplus U ( \varepsilon )$ associated with $U ( \varepsilon )$; it is denoted be $T ( \varepsilon )$.

Using the concept of the standard embedding Lie algebra $L ( \varepsilon ) = \operatorname { Inn } \operatorname { Der } T ( \varepsilon ) \oplus T ( \varepsilon )$ associated with a Lie triple system $T ( \varepsilon )$, one can obtain the construction of $L ( \varepsilon )$ associated with a Freudenthal–Kantor triple system $U ( \varepsilon )$. In fact, put

$L_{2}$ equal to the linear span of the endomorphisms

\begin{equation*} \left( \begin{array} { c c } { 0 } & { K ( a , b ) } \\ { 0 } & { 0 } \end{array} \right); \end{equation*}

$L _ { 1 } : = U ( \varepsilon ) \oplus ( 0 )$;

$L_{ - 1} : = ( 0 ) \oplus U ( \varepsilon )$;

$L_0$ equal to the linear span of the endomorphisms

\begin{equation*} \left( \begin{array} { c c } { L ( a , b ) } & { 0 } \\ { 0 } & { \varepsilon L ( b , a ) } \end{array} \right); \end{equation*}

$L_{ - 2}$ equal to the linear span of the endomorphisms

\begin{equation*} \left( \begin{array} { r r } { 0 } & { 0 } \\ { - \varepsilon K ( c , d ) } & { 0 } \end{array} \right). \end{equation*}

Then one obtains the decomposition

\begin{equation*} L ( \varepsilon ) = L _ { - 2 } \bigoplus L _ { - 1 } \bigoplus L _ { 0 } \bigoplus L _ { 1 } \bigoplus L _ { 2 }, \end{equation*}

and, more precisely,

\begin{equation*} \left[ \left( \begin{array} { c c } { \text{Id} } & { 0 } \\ { 0 } & { - \text{Id} } \end{array} \right) , L _ { i } \right] = i L _ { i } ( - 2 \leq i \leq 2 ). \end{equation*}

These results imply the dimensional formula

\begin{equation*} \operatorname { dim } _ { \Phi } L ( \varepsilon ) = 2 ( \operatorname { dim } _ { \Phi } U ( \varepsilon ) + \operatorname { dim } _ { \Phi } \{ K ( x , y ) \} _ { \operatorname { span } } )+ \end{equation*}

\begin{equation*} + \operatorname { dim } _ { \Phi } \{ L ( x , y ) \} _ { \operatorname { span } } = \end{equation*}

\begin{equation*} = \operatorname { dim } _ { \Phi } T ( \varepsilon ) + \operatorname { dim } _ { \Phi } \operatorname { Inn } \operatorname { Der } T ( \varepsilon ). \end{equation*}

This algebra $L ( \varepsilon )$ is called the Lie algebra associated with $U ( \varepsilon )$.

The concepts of a triple system and a supertriple system are important in the theory of quarks and Yang–Baxter equations.

Note that a "triple system" in the sense discussed above is totally different from "triple system" in combinatorics (see, e.g., Steiner triple system).

#### References

 [a1] H. Freudenthal, "Beziehungen der $E _ { 7 }$ und $E _ { 8 }$ zur Oktavenebene I–II" Indag. Math. , 16 (1954) pp. 218–230; 363–386 [a2] N. Kamiya, "The construction of all simple Lie algebras over $C$ from balanced Freudenthal–Kantor triple systems" , Contributions to General Algebra , 7 , Hölder–Pichler–Tempsky, Wien (1991) pp. 205–213 [a3] N. Kamiya, "On Freudenthal–Kantor triple systems and generalized structurable algebras" , Non-Associative Algebra and Its Applications , Kluwer Acad. Publ. (1994) pp. 198–203 [a4] N. Kamiya, S. Okubo, "On $\delta$-Lie supertriple systems associated with $( \varepsilon , \delta )$-Freudenthal–Kantor supertriple systems" Proc. Edinburgh Math. Soc. , 43 (2000) pp. 243–260 [a5] I.L. Kantor, "Models of exceptional Lie algebras" Soviet Math. Dokl. , 14 (1973) pp. 254–258 [a6] S. Okubo, "Introduction to octonion and other non-associative algebras in physics" , Cambridge Univ. Press (1995) [a7] K. Yamaguti, "On the metasymplectic geometry and triple systems" Surikaisekikenkyusho Kokyuroku, Res. Inst. Math. Sci. Kyoto Univ. , 306 (1977) pp. 55–92 (In Japanese)
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Freudenthal–Kantor triple system. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Freudenthal%E2%80%93Kantor_triple_system&oldid=22472