On the structure of semifields and lattice-ordered groups. (English) Zbl 0896.12001

Weinert, H. J.; Wiegandt, R.

Period. Math. Hung. 32, No. 1-2, 129-147 (1996).

The authors continue their study of the radical theory of semifields in their previous article [Commun. Algebra 20, 2419-2458 (1992; Zbl 0771.16016)]. They prove several interesting structure theorems for semisimple classes and radical classes of semifields that they apply to lattice ordered groups.
We quote only a very small number of these results. A semifield is a universal algebra \(\langle A;+, \cdot\rangle\) where \(\langle A;\cdot\rangle\) is a group with multiplicative identity \(e\), \(\langle A;+\rangle\) is a semigroup and \(\cdot\) left and right distributes over \(+\). The kernels of homomorphisms are determined by their \(e\)-classes which are normal subgroups \(K\) of \(\langle A;\cdot\rangle\) for which for any \(s,t \in A\), \(h\in K\), \(s+t\in K\) implies \(s+th\in K\). Addition in \(A/K\) is idempotent iff \(K\) is a subsemifield of \(\langle A;+,\cdot\rangle\). Let \({\mathfrak S}^*\) be the class of all semifields and let \(\mathfrak G\) be the class of all groups. The authors’ Kurosh-Amitsur radical theory for semifields has been developed within the category \({\mathfrak S}^* \cup \mathfrak G\) where the morphisms \(\phi :A \rightarrow B\) satisfy \(\phi \) is a semifield (group) homomorphism if \(A,B \in {\mathfrak S}^*\) (\(A,B \in \mathfrak G\)) and \(\phi \) preserves multiplication if \(A\in \mathfrak G , B\in \mathfrak S^*\). Radical classes and the corresponding semisimple classes are defined and the theory contains the Kurosh-Amitsur theory for groups. It is shown that under some natural conditions on a class \(M\), the class of all subdirect products of members of \(M\) is semisimple. The authors also study simple and subdirectly irreducible semifields.
Theorem 1: Let \(A\) be a semifield and let \(K\) be a kernel of \(A\) which is also a semifield. Set \(B=A/K\). Then \(\langle A;\cdot\rangle\) is isomorphic to a Schreier extension \(B\circ K\) with the property that the automorphism systems \({\alpha }^b\) are actually semifield automorphisms of \(K\).
The authors give a construction of semifields similar to the theorem which leads in the case when \(K\) is simple to a subdirectly irreducible semifield \(A\) whose heart is \(K\).
A lattice ordered group can be viewed as a semifield whose additive structure is a semilattice. The kernels of such semifields are semifields and correspond to the convex normal subgroups. Theorem 2: If \(R\) is a kernel in a commutative lattice ordered group \(A\), then \(R\) is closed in \(A\). If \(A\) is a complete lattice ordered group, then every kernel of \(A\) is a direct factor.

Reviewer: A.A.Iskander (Lafayette)

MSC:

12K10	Semifields
06F15	Ordered groups
20F60	Ordered groups (group-theoretic aspects)
06F20	Ordered abelian groups, Riesz groups, ordered linear spaces

Keywords:

semifields; semisimple classes; radical classes; lattice ordered groups

Citations:

Zbl 0771.16016

References:

[1]	G. Birkhoff,Lattice theory, AMS Coll. Publ. XXV, 3rd edition, Providence, Rhode Island, 1967. · Zbl 0153.02501
[2]	L.Fuchs,Teilweise geordnete algebraische Strukturen, Vandenhoek & Ruprecht, Göttingen, 1966. · Zbl 0154.00708
[3]	H. C.Hutchins, H.J.Weinert, Homomorphisms and kernels of semifields,Period. Math. Hungar. 21 (1990), 113–152. · Zbl 0718.16041 · doi:10.1007/BF01946851
[4]	L.Rédei,Algebra, Geest & Portig K.-G., Leipzig, 1959.
[5]	J.R.Teller, On the extensions of lattice-ordered groups,Pacific J. Math. 14 (1964), 709–718. · Zbl 0122.27904
[6]	H. J.Weinert, Über Halbringe und Halbkörper, I,Acta Math. Acad. Sci. Hungar. 13 (1962), 365–378. · Zbl 0125.01002 · doi:10.1007/BF02020799
[7]	H.J.Weinert, Ein Struktursatz für idempotente Halbkörper,Acta Math. Acad. Sci. Hungar. 15 (1964), 289–295. · Zbl 0142.00601 · doi:10.1007/BF01897137
[8]	H.J.Weinert, R.Wiegandt, A Kurosh-Amitsur radical theory for proper semifields,Comm. in Algebra 20 (1992), 2419–2458. · Zbl 0771.16016 · doi:10.1080/00927879208824471