## An Elementary Introduction to Groups and Representations by Hall B.C.

By Hall B.C.

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The contents of this e-book were used in classes given by way of the writer. the 1st was once a one-semester direction for seniors on the college of British Columbia; it used to be transparent that stable undergraduates have been completely in a position to dealing with effortless staff idea and its software to easy quantum chemical difficulties.

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The Lie algebra g of a matrix Lie group G is a real Lie algebra. Proof. 16, g is a real subalgebra of gl(n; C) complex matrices, and is thus a real Lie algebra. 31 (Ado). Every finite-dimensional real Lie algebra is isomorphic to a subalgebra of gl(n; R). Every finite-dimensional complex Lie algebra is isomorphic to a (complex) subalgebra of gl(n; C). This remarkable theorem is proved in Varadarajan. The proof is well beyond the scope of this course (which is after all a course on Lie groups), and requires a deep understanding of the structure of complex Lie algebras.

But as t varies from 0 to 1, etX is a continuous path connecting the identity to eX . 15. Let G be a matrix Lie group, with Lie algebra g. Let X be an element of g, and A an element of G. Then AXA−1 is in g. Proof. 3, et(AXA −1 ) = AetX A−1 , and AetX A−1 ∈ G. 16. Let G be a matrix Lie group, g its Lie algebra, and X, Y elements of g. Then 1. sX ∈ g for all real numbers s, 2. X + Y ∈ g, 3. XY − Y X ∈ g. If you are following the physics convention for the definition of the Lie algebra, then condition 3 should be replaced with the condition −i (XY − Y X) ∈ g.

E(α+β)X = eαX eβX for all real or complex numbers α, β. 4. If XY = Y X, then eX+Y = eX eY = eY eX . −1 5. If C is invertible, then eCXC = CeX C −1. 6. eX ≤ e X . It is not true in general that eX+Y = eX eY , although by 4) it is true if X and Y commute. This is a crucial point, which we will consider in detail later. ) Proof. Point 1) is obvious. Points 2) and 3) are special cases of point 4). To verify point 4), we simply multiply power series term by term. ) Thus eX eY = I +X+ X2 + ··· 2! I +Y + Y2 +··· 2!