• Source: Loewy ring

In mathematics, a left (right) Loewy ring or left (right) semi-Artinian ring is a ring in which every non-zero left (right) module has a non-zero socle, or equivalently if the Loewy length of every left (right) module is defined. The concepts are named after Alfred Loewy.




Loewy length


The Loewy length and Loewy series were introduced by Emil Artin, Cecil J. Nesbitt, and Robert M. Thrall (1944).
If M is a module, then define the Loewy series Mα for ordinals α by M0 = 0, Mα+1/Mα = socle(M/Mα), and Mα = ∪λ<α Mλ if α is a limit ordinal. The Loewy length of M is defined to be the smallest α with M = Mα, if it exists.




Semiartinian modules











R


M


{\displaystyle {}_{R}M}

is a semiartinian module if, for all epimorphisms



M
→
N


{\displaystyle M\rightarrow N}

, where



N
≠
0


{\displaystyle N\neq 0}

, the socle of



N


{\displaystyle N}

is essential in



N
.


{\displaystyle N.}


Note that if








R


M


{\displaystyle {}_{R}M}

is an artinian module then








R


M


{\displaystyle {}_{R}M}

is a semiartinian module. Clearly 0 is semiartinian.
If



0
→

M
′

→
M
→

M
″

→
0


{\displaystyle 0\rightarrow M'\rightarrow M\rightarrow M''\rightarrow 0}

is exact then




M
′



{\displaystyle M'}

and




M
″



{\displaystyle M''}

are semiartinian if and only if



M


{\displaystyle M}

is semiartinian.
If



{

M

i



}

i
∈
I




{\displaystyle \{M_{i}\}_{i\in I}}

is a family of



R


{\displaystyle R}

-modules, then




⊕

i
∈
I



M

i




{\displaystyle \oplus _{i\in I}M_{i}}

is semiartinian if and only if




M

j




{\displaystyle M_{j}}

is semiartinian for all



j
∈
I
.


{\displaystyle j\in I.}





Semiartinian rings






R


{\displaystyle R}

is called left semiartinian if






R


R


{\displaystyle _{R}R}

is semiartinian, that is,



R


{\displaystyle R}

is left semiartinian if for any left ideal



I


{\displaystyle I}

,



R

/

I


{\displaystyle R/I}

contains a simple submodule.
Note that



R


{\displaystyle R}

left semiartinian does not imply that



R


{\displaystyle R}

is left artinian.




References



Assem, Ibrahim; Simson, Daniel; Skowroński, Andrzej (2006), Elements of the representation theory of associative algebras. Vol. 1: Techniques of representation theory, London Mathematical Society Student Texts, vol. 65, Cambridge: Cambridge University Press, ISBN 0-521-58631-3, Zbl 1092.16001
Artin, Emil; Nesbitt, Cecil J.; Thrall, Robert M. (1944), Rings with Minimum Condition, University of Michigan Publications in Mathematics, vol. 1, Ann Arbor, MI: University of Michigan Press, MR 0010543, Zbl 0060.07701
Nastasescu, Constantin; Popescu, Nicolae (1968), "Anneaux semi-artiniens", Bulletin de la Société Mathématique de France, 96: 357–368, ISSN 0037-9484, MR 0238887, Zbl 0227.16014
Nastasescu, Constantin; Popescu, Nicolae (1966), "Sur la structure des objets de certaines catégories abéliennes", Comptes Rendus de l'Académie des Sciences, Série A, 262, GAUTHIER-VILLARS/EDITIONS ELSEVIER 23 RUE LINOIS, 75015 PARIS, FRANCE: A1295–A1297

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