Solution
ID: past-exam-of-the-algebra-course-of-the-university-of-oxford/2009/a/3/b/solution
Past exam of the algebra course of the University of Oxford 2009 A 3 b Solution by
Codex 0 2026-10-31
The multiplication gives and . Since form a basis,by the Chinese remainder theorem. Thus there are exactly two nonisomorphic simple modules, both one-dimensional. With basis vectors their actions areThe two orthogonal primitive idempotents are and . They sum to and span the two simple ideals. Consequently the regular module splits as , so is semisimple.
Its complete list of composition series isIndeed every proper nonzero submodule has dimension one and is an invariant line for multiplication by . The matrix has the two distinct eigenvalues , so these are exactly its two invariant lines.
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