Reduced, regular and smooth are different properties
Research context
postgraduate concept · established · As of 2026-10-10
A ring is reduced when it has no nonzero nilpotents.
Key knowledge
A ring is reduced when it has no nonzero nilpotents. A Noetherian local ring is regular when its Krull dimension equals the dimension of its maximal ideal modulo its square over the residue field. A scheme smooth over a field has a relative geometric property; regularity agrees with smoothness for finite-type schemes over a perfect field, but this implication can fail over imperfect fields.
Values and equations
dim A = dim_(A/m)(m/m²) for a regular Noetherian local ring (A,m)
- dim A
- Krull dimension
- m
- Maximal ideal
- A/m
- Residue field
Conditions and scope
- Regularity uses a Noetherian local ring and its residue field.
- The perfect-field and finite-type hypotheses are essential to the stated regular/smooth bridge; all residue field extensions over a perfect field are separable, enabling Lemma10.140.5. Reducedness alone is weaker.
- For k=F_p(t), Spec(k[x]/(x^p−t)) is a regular field scheme but becomes nonreduced after adjoining t^(1/p), so it is not geometrically regular or smooth over k.
Sources · 5
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
The Stacks Project authors4 sources
- Algebraic GeometryJ. S. MilneTextbook chapter · PDF
Evidence and review
Author checked this English statement, displayed equation where present and stated hypotheses against these source locations: Section 10.60: Krull dimension and definition of regular local ring; Chapters 2 and 4: Zariski topology, strong Nullstellensatz Theorem 2.16, coordinate rings and nonsingular points; Definition33.12.1, Lemmas33.12.2/33.12.6: finite-type smoothness/geometric regularity; Example33.12.7: regular imperfect-field examples failing geometric regularity; Definition10.45.1 and Lemma10.45.2: all field extensions separable over a perfect field and characteristic-zero/p-th-root criterion; Lemma10.140.5 and proof: smoothness at a prime iff regular local ring when the residue field is separable over the base. The check is bounded to the recorded claims; no complete proof reconstruction, software proof replay or universal literature completeness is claimed.
2026-10-10 · Author source-body/statement/equation/domain check, with graph associations scoped individually. Separate independent content/source/inventory review is required before branch closure.
Entry details
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