Paper 1, Section II, H

Representation Theory | Part II, 2014

(i) Let KK be any field and let λ∈K\lambda \in K. Let Jλ,nJ_{\lambda, n} be the n×nn \times n Jordan block

Jλ,n=(λ10⋯00⋱⋱⋮⋮⋱⋱0⋮⋱10⋯⋯0λ)J_{\lambda, n}=\left(\begin{array}{ccccc} \lambda & 1 & 0 & \cdots & 0 \\ 0 & \ddots & \ddots & & \vdots \\ \vdots & & \ddots & \ddots & 0 \\ \vdots & & & \ddots & 1 \\ 0 & \cdots & \cdots & 0 & \lambda \end{array}\right)

Compute Jλ,nrJ_{\lambda, n}^{r} for each r⩾0r \geqslant 0.

(ii) Let GG be a cyclic group of order NN, and let KK be an algebraically closed field of characteristic p⩾0p \geqslant 0. Determine all the representations of GG on vector spaces over KK, up to equivalence. Which are irreducible? Which do not split as a direct sum W⊕W′W \oplus W^{\prime}, with W≠0W \neq 0 and W′≠0?W^{\prime} \neq 0 ?

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