Rings, ideals and modules
| English | Français |
|---|---|
| ideal/aɪˈdɪəl/ | ideal |
| torsion/ˈtɔːʃn/ | torsion |
A decision before an answer
- In arithmetic modulo 6, two nonzero values multiply to zero. Cancelling either one would erase valid solutions.
- Your goal: Distinguish units, zero divisors and integral domains.
Read the relationship
- For the domain, ideal and quotient examples below, use a commutative ring with identity 1 distinct from 0. A source problem may specify a general ring instead; do not assume its multiplication commutes unless stated or proved. A unit has a multiplicative inverse. A nonzero zero divisor multiplies some nonzero element to zero. An integral domain has no such zero divisors; cancellation of a nonzero factor then works. A field is a domain in which every nonzero element is a unit. Z is a domain but not a field; Z/6Z is neither.
- Identify ideals and interpret quotient rings.
How many units are in Z/10Z?
Only 1,3,7,9 are coprime to 10; each has a multiplicative inverse.
Use the defining rule
- An ideal I is an additive subgroup that absorbs multiplication by every ring element. This is stronger than being a subring. In Z, nZ is an ideal; quotient elements are integer residue classes modulo n. In a commutative ring, R/I is a field exactly when I is maximal, and it is a domain exactly when I is prime. The ideal must be proper in both statements.
- Compare modules over rings with vector spaces over fields.
Which integer ideal is maximal in Z?
Z/7Z is a field because 7 is prime. The composite quotients have zero divisors, and Z/0Z is Z, not a field.
Check the conditions
- A module allows scalars from a ring instead of requiring a field. Every abelian group is a Z-module by repeated addition, but it need not have a vector-space basis. In Z/6Z as a Z-module, 6 times the nonzero residue 1 is zero; this is torsion. For a vector space over a field, a nonzero scalar is invertible and cannot annihilate a nonzero vector.
- Derive Boolean-ring properties without assuming commutativity.
In Z/6Z, 2·3=0, so 2 and 3 are zero divisors. The units are 1 and 5 because their gcd with 6 is 1. In Z, the ideal 5Z gives the field Z/5Z, while 6Z gives a quotient with zero divisors. As Z-modules, the map from Z to Z/6Z has kernel 6Z; the quotient identifies integers differing by a multiple of 6, rather than producing a real vector space.
In Z/8Z, the least positive integer k for which k times the residue 2 is zero is ____.
2k is divisible by 8 first at k=4.
Apply the task format
- A submodule is closed under addition and all permitted scalar actions. A linear map of modules preserves both. The kernel and image are submodules, and the quotient by the kernel is isomorphic to the image. Do not apply finite-dimensional rank-nullity to an arbitrary module without establishing an appropriate free-module setting; integer row operations and field row operations permit different divisions. In a general Boolean ring, every a satisfies a²=a. Do not assume commutativity to prove it: idempotence of a+a gives 4a=2a, hence 2a=0. Expanding (a+b)²=a+b gives ab+ba=0, and characteristic two makes −ba=ba; therefore ab=ba. Idempotence does not imply nilpotence: in F2, the nonzero element 1 satisfies 1^n=1 for every positive n.
- Derive Boolean-ring properties without assuming commutativity.
A set can be a submodule without being a vector space over the rationals. Scalar division is valid only when the scalar inverse belongs to the structure.
Which answer fits this case?
Distinguish units, zero divisors and integral domains
Every integral domain is a field.
Z has cancellation but the nonzero integer 2 has no inverse in Z.
Keep the distinctions
- ideal 理想 — An additive subgroup of a ring absorbing multiplication by all ring elements.
- torsion 挠性 — An element is annihilated by a nonzero scalar in the stated module.
- Distinguish units, zero divisors and integral domains.
- Identify ideals and interpret quotient rings.
- Compare modules over rings with vector spaces over fields.
- Derive Boolean-ring properties without assuming commutativity.
Match each term with its precise meaning in this lesson.
Keep the distinctions stated in the teaching example.
Put this lesson’s reasoning or event sequence in order.
The order follows the stated process; check each stage before the next.