B3.5

Representation Theory
Part II, 2004

Compute the character table for the group A5A_{5} of even permutations of five elements. You may wish to follow the steps below.

(a) List the conjugacy classes in A5A_{5} and their orders.

(b) A5A_{5} acts on C5\mathbb{C}^{5} by permuting the standard basis vectors. Show that C5\mathbb{C}^{5} splits as CV\mathbb{C} \oplus V, where C\mathbb{C} is the trivial 1-dimensional representation and VV is irreducible.

(c) By using the formula for the character of the symmetric square S2VS^{2} V,

χS2V(g)=12[χV(g)2+χV(g2)]\chi_{S^{2} V}(g)=\frac{1}{2}\left[\chi_{V}(g)^{2}+\chi_{V}\left(g^{2}\right)\right]

decompose S2VS^{2} V to produce a 5-dimensional, irreducible representation, and find its character.

(d) Show that the exterior square Λ2V\Lambda^{2} V decomposes into two distinct irreducibles and compute their characters, to complete the character table of A5A_{5}.

[Hint: You can save yourself some computational effort if you can explain why the automorphism of A5A_{5}, defined by conjugation by a transposition in S5S_{5}, must swap the two summands of Λ2V\Lambda^{2} V.]