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In mathematics, **F _{4}** is the name of a Lie group and also its Lie algebra

- Algebra
- Dynkin diagram
- Weyl/Coxeter group
- Cartan matrix
- F4 lattice
- Roots of F4
- F4 polynomial invariant
- Representations
- See also
- References

The compact real form of F_{4} is the isometry group of a 16-dimensional Riemannian manifold known as the octonionic projective plane **OP**^{2}. This can be seen systematically using a construction known as the *magic square*, due to Hans Freudenthal and Jacques Tits.

There are 3 real forms: a compact one, a split one, and a third one. They are the isometry groups of the three real Albert algebras.

The F_{4} Lie algebra may be constructed by adding 16 generators transforming as a spinor to the 36-dimensional Lie algebra **so**(9), in analogy with the construction of E_{8}.

In older books and papers, F_{4} is sometimes denoted by E_{4}.

The Dynkin diagram for F_{4} is: .

Its Weyl/Coxeter group *G* = *W*(F_{4}) is the symmetry group of the 24-cell: it is a solvable group of order 1152. It has minimal faithful degree *μ*(*G*) = 24,^{ [1] } which is realized by the action on the 24-cell.

The F_{4} lattice is a four-dimensional body-centered cubic lattice (i.e. the union of two hypercubic lattices, each lying in the center of the other). They form a ring called the Hurwitz quaternion ring. The 24 Hurwitz quaternions of norm 1 form the vertices of a 24-cell centered at the origin.

The 48 root vectors of F_{4} can be found as the vertices of the 24-cell in two dual configurations, representing the vertices of a disphenoidal 288-cell if the edge lengths of the 24-cells are equal:

**24-cell vertices:**

- 24 roots by (±1, ±1, 0, 0), permuting coordinate positions

**Dual 24-cell vertices:**

- 8 roots by (±1, 0, 0, 0), permuting coordinate positions
- 16 roots by (±1/2, ±1/2, ±1/2, ±1/2).

One choice of simple roots for F_{4}, , is given by the rows of the following matrix:

Just as O(*n*) is the group of automorphisms which keep the quadratic polynomials *x*^{2} + *y*^{2} + ... invariant, F_{4} is the group of automorphisms of the following set of 3 polynomials in 27 variables. (The first can easily be substituted into other two making 26 variables).

Where *x*, *y*, *z* are real-valued and *X*, *Y*, *Z* are octonion valued. Another way of writing these invariants is as (combinations of) Tr(*M*), Tr(*M*^{2}) and Tr(*M*^{3}) of the hermitian octonion matrix:

The set of polynomials defines a 24-dimensional compact surface.

The characters of finite dimensional representations of the real and complex Lie algebras and Lie groups are all given by the Weyl character formula. The dimensions of the smallest irreducible representations are (sequence A121738 in the OEIS ):

- 1, 26, 52, 273, 324, 1053 (twice), 1274, 2652, 4096, 8424, 10829, 12376, 16302, 17901, 19278, 19448, 29172, 34749, 76076, 81081, 100776, 106496, 107406, 119119, 160056 (twice), 184756, 205751, 212992, 226746, 340119, 342056, 379848, 412776, 420147, 627912…

The 52-dimensional representation is the adjoint representation, and the 26-dimensional one is the trace-free part of the action of F_{4} on the exceptional Albert algebra of dimension 27.

There are two non-isomorphic irreducible representations of dimensions 1053, 160056, 4313088, etc. The fundamental representations are those with dimensions 52, 1274, 273, 26 (corresponding to the four nodes in the Dynkin diagram in the order such that the double arrow points from the second to the third).

In mathematics, the **octonions** are a normed division algebra over the real numbers, a kind of hypercomplex number system. The octonions are usually represented by the capital letter O, using boldface **O** or blackboard bold . Octonions have eight dimensions; twice the number of dimensions of the quaternions, of which they are an extension. They are noncommutative and nonassociative, but satisfy a weaker form of associativity; namely, they are alternative. They are also power associative.

In group theory, the **quaternion group** Q_{8} (sometimes just denoted by Q) is a non-abelian group of order eight, isomorphic to the eight-element subset of the quaternions under multiplication. It is given by the group presentation

In mechanics and geometry, the **3D rotation group**, often denoted **SO(3)**, is the group of all rotations about the origin of three-dimensional Euclidean space under the operation of composition.

In mathematics, the **special unitary group** of degree *n*, denoted SU(*n*), is the Lie group of *n* × *n* unitary matrices with determinant 1.

In physics and mathematics, the **Lorentz group** is the group of all Lorentz transformations of Minkowski spacetime, the classical and quantum setting for all (non-gravitational) physical phenomena. The Lorentz group is named for the Dutch physicist Hendrik Lorentz.

In mathematics, a **root system** is a configuration of vectors in a Euclidean space satisfying certain geometrical properties. The concept is fundamental in the theory of Lie groups and Lie algebras, especially the classification and representation theory of semisimple Lie algebras. Since Lie groups and Lie algebras have become important in many parts of mathematics during the twentieth century, the apparently special nature of root systems belies the number of areas in which they are applied. Further, the classification scheme for root systems, by Dynkin diagrams, occurs in parts of mathematics with no overt connection to Lie theory. Finally, root systems are important for their own sake, as in spectral graph theory.

In the mathematical field of Lie theory, a **Dynkin diagram**, named for Eugene Dynkin, is a type of graph with some edges doubled or tripled. Dynkin diagrams arise in the classification of semisimple Lie algebras over algebraically closed fields, in the classification of Weyl groups and other finite reflection groups, and in other contexts. Various properties of the Dynkin diagram correspond to important features of the associated Lie algebra.

In mathematics, a **simple Lie group** is a connected non-abelian Lie group *G* which does not have nontrivial connected normal subgroups. The list of simple Lie groups can be used to read off the list of simple Lie algebras and Riemannian symmetric spaces.

In mathematics, **G _{2}** is the name of three simple Lie groups (a complex form, a compact real form and a split real form), their Lie algebras as well as some algebraic groups. They are the smallest of the five exceptional simple Lie groups. G

In mathematics, **E _{6}** is the name of some closely related Lie groups, linear algebraic groups or their Lie algebras , all of which have dimension 78; the same notation E

In mathematics, the **Heisenberg group**, named after Werner Heisenberg, is the group of 3×3 upper triangular matrices of the form

In mathematics, **E _{8}** is any of several closely related exceptional simple Lie groups, linear algebraic groups or Lie algebras of dimension 248; the same notation is used for the corresponding root lattice, which has rank 8. The designation E

In mathematics, **E _{7}** is the name of several closely related Lie groups, linear algebraic groups or their Lie algebras

In mathematics, **SO(8)** is the special orthogonal group acting on eight-dimensional Euclidean space. It could be either a real or complex simple Lie group of rank 4 and dimension 28.

In mathematics, a **reductive group** is a type of linear algebraic group over a field. One definition is that a connected linear algebraic group *G* over a perfect field is reductive if it has a representation with finite kernel which is a direct sum of irreducible representations. Reductive groups include some of the most important groups in mathematics, such as the general linear group *GL*(*n*) of invertible matrices, the special orthogonal group *SO*(*n*), and the symplectic group *Sp*(2*n*). **Simple algebraic groups** and **semisimple algebraic groups** are reductive.

In mathematics, the **split-octonions** are an 8-dimensional nonassociative algebra over the real numbers. Unlike the standard octonions, they contain non-zero elements which are non-invertible. Also the signatures of their quadratic forms differ: the split-octonions have a split signature (4,4) whereas the octonions have a positive-definite signature (8,0).

Many branches of mathematics study objects of a given type and prove a classification theorem. A common theme is that the classification results in a number of series of objects and a finite number of exceptions — often with desirable properties — that do not fit into any series. These are known as **exceptional objects**. In many cases, these exceptional objects play a further and important role in the subject. Furthermore, the exceptional objects in one branch of mathematics often relate to the exceptional objects in others.

In mathematics, an **Albert algebra** is a 27-dimensional exceptional Jordan algebra. They are named after Abraham Adrian Albert, who pioneered the study of non-associative algebras, usually working over the real numbers. Over the real numbers, there are three such Jordan algebras up to isomorphism. One of them, which was first mentioned by Pascual Jordan, John von Neumann, and Eugene Wigner (1934) and studied by Albert (1934), is the set of 3×3 self-adjoint matrices over the octonions, equipped with the binary operation

In mathematics, the **classical groups** are defined as the special linear groups over the reals **R**, the complex numbers **C** and the quaternions **H** together with special automorphism groups of symmetric or skew-symmetric bilinear forms and Hermitian or skew-Hermitian sesquilinear forms defined on real, complex and quaternionic finite-dimensional vector spaces. Of these, the **complex classical Lie groups** are four infinite families of Lie groups that together with the exceptional groups exhaust the classification of simple Lie groups. The **compact classical groups** are compact real forms of the complex classical groups. The finite analogues of the classical groups are the **classical groups of Lie type**. The term "classical group" was coined by Hermann Weyl, it being the title of his 1939 monograph *The Classical Groups*.

- Adams, J. Frank (1996).
*Lectures on exceptional Lie groups*. Chicago Lectures in Mathematics. University of Chicago Press. ISBN 978-0-226-00526-3. MR 1428422. - John Baez,
*The Octonions*, Section 4.2: F_{4}, Bull. Amer. Math. Soc.**39**(2002), 145-205. Online HTML version at http://math.ucr.edu/home/baez/octonions/node15.html. - Chevalley C, Schafer RD (February 1950). "The Exceptional Simple Lie Algebras F(4) and E(6)".
*Proc. Natl. Acad. Sci. U.S.A*.**36**(2): 137–41. Bibcode:1950PNAS...36..137C. doi: 10.1073/pnas.36.2.137 . PMC 1063148 . PMID 16588959. - Jacobson, Nathan (1971-06-01).
*Exceptional Lie Algebras*(1st ed.). CRC Press. ISBN 0-8247-1326-5.

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