Cohen A.'s Group theory for Maths, Physics and Chemistry students PDF

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3. For which pairs (l, m) are the representations rl and rm equivalent? 13. 10. Show that ρa ∼ ρb if and only if ab = 0. 9, we saw that V ∗ is a invariant under the action of G on F (V ). This gives rise to the following definition. 52 CHAPTER 4. 14. Let r be a representation of a group G on V . A subspace W is invariant if r(g)W ⊂ W for all g ∈ G. In this case, r|W : G → GL(W ) given by r|W (g) = r(g)|W , is called a subrepresentation of r, and W is called a sub-G-module of V . If G is clear from the context, we leave it out and call W a submodule of V .

Set a := g(0). Then ta−1 g(0) = 0, so t−1 a g is an isometry fixing 0, and it is clear that a = g(0) is the only value for which this is the case. 4. Let r ∈ AO(n, R) be an isometry fixing 0. Then r is an orthogonal R-linear map. Proof. Let x, y ∈ Rn , and compute (rx, ry) = (−d(rx, ry)2 +d(rx, 0)2 +d(ry, 0)2 )/2 = (−d(x, y)2 +d(x, 0)2 +d(y, 0)2 )/2 = (x, y), where we use that r is an isometry fixing 0. Hence, r leaves the inner product invariant. This proves the second statement, if we take the first statement for granted.

For a finite group G, every finite-dimensional G-module is completely reducible. Proof. Proceed by induction. Suppose that the stament holds for all representations of dimension smaller than n, and let V be an n-dimensional representation. If V is irreducible, then we are done. Otherwise there exists a sub-G-module U with 0 U V . 19, U has a invariant complementary sub-Gmodule W . As both dim U and dim W are smaller than n, we may apply the induction hypothesis, and find that U is the direct sum of irreducible sub-Gmodules U1 , .

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Group theory for Maths, Physics and Chemistry students by Cohen A.


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