# 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 by 2 matrix *M* over the ring such that *(a b)M=(d 0)*. (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 *(b _{1},...,b_{n})* of elements of the ring which generate it as a right module (i.e.,

*b*) can be completed to a (not necessarily square) invertible matrix by adding some number of rows. (The criterion of being

_{1}R+...+b_{n}R=R**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 IBN.

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 *R*[*x*] is 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, MR 1646314 - 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**: 464–491, doi:10.2307/1990591, ISSN 0002-9947, JSTOR 1990591, MR 0031470 - 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, MR 0485842 - 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, MR 0186687