Seminormal ring

In algebra, a seminormal ring is a commutative reduced ring in which, whenever x, y satisfy $x^3=y^2$, there is s with $s^2=x$ and $s^3=y$. This definition was given by (Swan 1980) as a simplification of the original definition of (Traverso 1970). A basic example is an integrally closed domain, i.e., a normal ring. For an example which is not normal, one can consider the non-integral ring $\mathbb{\mathbb{Z}}[x,y]/xy$, or the ring of a nodal curve.

In general, a reduced scheme $X$ can be said to be seminormal if every morphism $Y\to X$ which induces a homeomorphism of topological spaces, and an isomorphism on all residue fields, is an isomorphism of schemes.

A semigroup is said to be seminormal if its semigroup algebra is seminormal.

References

• Swan, Richard G. (1980), "On seminormality", Journal of Algebra 67 (1): 210–229, doi:10.1016/0021-8693(80)90318-X, ISSN 0021-8693 
• Traverso, Carlo (1970), "Seminormality and Picard group", Ann. Scuola Norm. Sup. Pisa (3) 24: 585–595, http://www.numdam.org/item?id=ASNSP_1970_3_24_4_585_0 
• Vitulli, Marie A. (2011), "Weak normality and seminormality", Commutative algebra---Noetherian and non-Noetherian perspectives, Berlin, New York: Springer-Verlag, pp. 441–480, doi:10.1007/978-1-4419-6990-3_17, ISBN 978-1-4419-6989-7, http://pages.uoregon.edu/vitulli/WeakAndSeminormality.pdf 
• Charles Weibel, The K-book: An introduction to algebraic K-theory