# Representations of Groups: A Computational Approach by Klaus Lux

By Klaus Lux

The illustration conception of finite teams has obvious quick progress lately with the improvement of effective algorithms and computing device algebra platforms. this is often the 1st e-book to supply an creation to the standard and modular illustration thought of finite teams with exact emphasis at the computational features of the topic. Evolving from classes taught at Aachen college, this well-paced textual content is perfect for graduate-level research. The authors supply over two hundred routines, either theoretical and computational, and contain labored examples utilizing the pc algebra procedure hole. those make the summary concept tangible and interact scholars in actual hands-on paintings. hole is freely on hand from www.gap-system.org and readers can obtain resource code and options to chose workouts from the book's web content.

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Extra info for Representations of Groups: A Computational Approach

Example text

The map G · (x1 , g · y1 ) → HgU deﬁnes the second one-to-one correspondence. 12 If O is a G-orbit on X × Y and (x, y) ∈ O then O(x) = StabG (x) · y and O (y) = StabG (y) · x. Hence |O| = [G : StabG ((x, y))] = [G : StabG (x)]|O(x)| = [G : StabG (y)]|O (y)|. Hence, if Ω = X = Y is a transitive G-set, then |O(x)| = |O (y)| = |O| |Ω| for arbitrary (x, y) ∈ O. 11 is the same as the number of orbits of H = StabG (ω) on Ω for any ω ∈ Ω) is called the rank of Ω and also of KΩ. If the rank is 2 then one also says that G acts doubly transitively on Ω.

1 Let Ω be a ﬁnite non-empty set. Note that Ω or more precisely (Ω, ·) is called a (left) G-set, and G is said to act on Ω (from the left) if ·: G × Ω → Ω (g, ω) → g · ω is a map satisfying g1 · (g2 · ω) = (g1 g2 ) · ω 1G · ω = ω for all g1 , g2 ∈ G, ω ∈ Ω, for all ω ∈ Ω. 11) If Ω1 , Ω2 are G-sets then a map ϕ : Ω1 → Ω2 is called G-equivariant or a G-map if ϕ(g · ω) = g · ϕ(ω) for all g ∈ G , ω ∈ Ω1 . The set of G-maps from Ω1 to Ω2 is usually denoted by HomG (Ω1 , Ω2 ). If, in addition, ϕ is bijective then ϕ is called a G-isomorphism, and if such a ϕ exists Ω1 , Ω2 are called isomorphic G-sets; in symbols, Ω1 ∼ =G Ω2 .

6). There is a variant of the Meataxe algorithm due to Holt and Rees [86], which has a performance quite independent of the size of the ﬁnite ﬁeld K that we are going to present. It is based on the following lemma. 9 Let A be an algebra over a ﬁeld K and V be an A-module of ﬁnite dimension over K. Let f be an irreducible factor of the characteristic polynomial ca of δV (a) for some a ∈ A. Assume that deg f = dimK kerV (f (a)). Then V is a simple A-module if (and only if ) (a) A · v = V for some 0 = v ∈ kerV (f (a)) and (b) x · A = V for some 0 = x ∈ kerV (f (a)).