Hermite ring
In algebra, the term Hermite ring (after Charles Hermite) has been applied to three different objects. According to (Kaplansky 1949) (p. 465), a ring is right Hermite if, for every two elements a and b of the ring, there is an element d of the ring and an invertible 2×2 matrix M over the ring such that (a b)M = (d 0), and the term left Hermite is defined similarly. Matrices over such a ring can be put in Hermite normal form by right multiplication by a square invertible matrix ((Kaplansky 1949), p. 468.) (Lam 2006) (appendix to §I.4) calls this property K-Hermite, using Hermite instead in the sense given below.
According to (Lam 1978) (§I.4, p. 26), a ring is right Hermite if any finitely generated stably free right module over the ring is free. This is equivalent to requiring that any row vector (b1,...,bn) of elements of the ring which generate it as a right module (i.e., b1R + ... + bnR = R) can be completed to a (not necessarily square ) invertible matrix by adding some number of rows. The criterion of being left Hermite can be defined similarly. (Lissner 1965) (p. 528) earlier called a commutative ring with this property an H-ring.
According to (Cohn 2006) (§0.4), a ring is Hermite if, in addition to every stably free (left) module being free, it has invariant basis number.
All commutative rings which are Hermite in the sense of Kaplansky are also Hermite in the sense of Lam, but the converse is not necessarily true. All Bézout domains are Hermite in the sense of Kaplansky, and a commutative ring which is Hermite in the sense of Kaplansky is also a Bézout ring ((Lam 2006), pp. 39-40.)
The Hermite ring conjecture, introduced by (Lam 1978) (p. xi), states that if R is a commutative Hermite ring, then the polynomial ring R[x] is also a Hermite ring.
References
- Cohn, P. M. (2000), "From Hermite rings to Sylvester domains", Proceedings of the American Mathematical Society 128 (7): 1899–1904, doi:10.1090/S0002-9939-99-05189-8, ISSN 0002-9939
- Cohn, P. M. (2006), Free Ideal Rings and Localization in General Rings, Cambridge University Press, ISBN 9780521853378
- Kaplansky, Irving (1949), "Elementary divisors and modules", Transactions of the American Mathematical Society 66 (2): 464–491, doi:10.2307/1990591, ISSN 0002-9947
- Lam, T. Y. (1978), Serre's Conjecture, Lecture Notes in Mathematics, 635, Berlin, New York: Springer-Verlag, doi:10.1007/BFb0068340, ISBN 978-3-540-08657-4
- Lam, T. Y. (2006), Serre's Problem on Projective Modules, Springer Monographs in Mathematics, Berlin, Heidelberg: Springer-Verlag, doi:10.1007/978-3-540-34575-6, ISBN 978-3-540-23317-6
- Lissner, David (1965), "Outer product rings", Transactions of the American Mathematical Society 116: 526–535, doi:10.2307/1994132, ISSN 0002-9947
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