Modules: Linear Algebra Over a Ring · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Linear algebra with ring scalars

Mathematics · Abstract Algebra · ages 22-24
Name ______________________   Date ____________
  1. Free module of integer pairs. What is its rank?

    Answer: ______________

  2. Integers modulo 2 as a module over integers. Why does it have no basis?

    • Every element is killed by 2, so no linearly independent set exists
    • It has too many elements
    • It is a field under addition
    • It has basis {0}
  3. Integers mod 2 as a module over integers: why does it have no basis?

    • It has too many elements
    • Every element is killed by 2, so no independent set exists
    • It has basis containing 0
  4. Vector spaces always have bases, but modules over a ring need not.

    Circle one:   True   False

  5. A square complex matrix is similar to a Jordan form given by the structure theorem for polynomial modules.

    Circle one:   True   False

  6. A square complex matrix is similar to a Jordan form, which is the structure theorem for polynomial modules applied to complex n space. Is this viewpoint correct?

    Circle one:   True   False

  7. Which decomposition matches integers modulo 6 as an integers module?

    • Integers plus integers
    • Cyclic of order 6 plus integers
    • Cyclic of order 2 plus cyclic of order 3
    • Zero module
  8. Which decomposition matches integers mod 6 as a module over integers?

    • Cyclic of order 2 plus cyclic of order 3
    • Integers plus integers
    • Zero module
  9. A 2 by 2 matrix has only eigenvalue 5 and only one independent eigenvector. What is its Jordan form?

    • Diagonal with 5 and 5
    • Rotation matrix
    • Single block with 5 on the diagonal and 1 above
  10. Module direct sum of cyclic of order 4 with cyclic of order 6 has torsion size what?

    Answer: ______________

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Linear algebra with ring scalars W1-mt_0GMVp-s8wD-s1

  1. 2 · Pairs need 2 basis elements, (1, 0) and (0, 1), so rank is 2.
  2. Every element is killed by 2, so no linearly independent set exists · Every element is killed by 2, so no linearly independent set exists is correct, since 2 times any element is 0, blocking independence.
  3. Every element is killed by 2, so no independent set exists · The scalar 2 kills everything, which blocks linear independence.
  4. True · Division by scalars can fail over rings, so torsion can block bases.
  5. True · Polynomial rings are principal ideal domains, and the module structure gives Jordan blocks.
  6. True · Polynomial ring is a principal ideal domain, and its module structure gives Jordan blocks.
  7. Cyclic of order 2 plus cyclic of order 3 · Cyclic of order 2 plus cyclic of order 3 is correct by Chinese remainder, while the others mismatch rank or size.
  8. Cyclic of order 2 plus cyclic of order 3 · Coprime factors split, and 2 times 3 recovers size 6.
  9. Single block with 5 on the diagonal and 1 above · One eigenvector means one block of size 2.
  10. 24 · Total size 4 times 6 equals 24, all torsion.
Worksheet · LightMySky