What does G2 stand for? 

G2

In mathematics, G₂ is the name of three simple Lie groups, their Lie algebras, as well as some algebraic groups. They are the smallest of the five exceptional simple Lie groups. G₂ has rank 2 and dimension 14. It has two fundamental representations, with dimension 7 and 14. The compact form of G₂ can be described as the automorphism group of the octonion algebra or, equivalently, as the subgroup of SO that preserves any chosen particular vector in its 8-dimensional real spinor representation. Robert Bryant introduced the definition of G₂ as the subgroup of that preserves the non-degenerate 3-form with denoting In older books and papers, G₂ is sometimes denoted by E₂.

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