# Classification of Simple Modules of the Ore Extension \(K[X][Y; f\frac{d}{dX}]\)

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## Abstract

For the algebras \(\Lambda \) in the title of the paper, a classification of simple modules is given, an explicit description of the prime and completely prime spectra is obtained, the global and the Krull dimensions of \(\Lambda \) are computed.

## Keywords

A skew polynomial ring A prime ideal A completely prime ideal A simple module The global dimension The Krull dimension A normal element## Mathematics Subject Classification

16D60 13N10 16S32## 1 Introduction

Let *D* be a ring and \(A= D[x; \sigma , \delta ]\) be a skew polynomial ring where \(\sigma \) is an automorphism of *D* and \(\delta \) is a \(\sigma \)-derivation of *D* (for all \(a, b\in D\), \(\delta (ab) = \delta (a)b+\sigma (a) \delta (b)\)). The ring *A* is generated by *D* and *x* subject to the defining relations \(xa=\sigma (a) x+ \delta (a)\) for all elements \(a\in D\). When *D* is a Dedekind domain, a classification of simple *A*-modules is given in [4]. This is a large class of rings. A machinery is developed in [4] to cover all possible situations (non-commutative valuations, etc).

*A*where \(\sigma =\mathrm{id}\) is the identity automorphism of the polynomial ring

*K*[

*X*], \(f\in K[X]\) and \(\delta = f\frac{d}{dX}\) is a

*K*-derivation of

*K*[

*X*] (\(\delta (X)=f\)). If \(f=1\) (or, more generally, \(f\in K^\times \backslash \{ 0\}\)) then the algebra \(\Lambda (1)\) is the

*Weyl algebra*

Recently, classifications of simple *weight* modules are obtained for some classical algebras (the *Euclidean* algebra, the *Schrödinger* algebra, the universal enveloping algebra \(U(\mathrm{sl}_2\ltimes V_2)\)), see [5, 6, 7]. In these classifications, classifications of *all simple* modules over certain subalgebras of the Weyl algebra \(A_1\) that contain the polynomial algebra *K*[*X*] (the, so-called, *polyonic* algebras) play a crucial role. The polyonic algebras are investigated in [8]. Each polyonic algebra contains the algebra \(\Lambda =\Lambda (f)\) for some non-scalar polynomial \(f\in K[X]\) which play an important role in studying of its properties. This is the main reason why we decided to collect main properties of the algebras \(\Lambda \) in this paper. In particular, a classification of simple \(\Lambda \)-modules is given in Sect. 2 (Lemma 2.1 and Theorem 2.10). This classification can be derived from [4] but we give different and simpler proofs which are based on generalized Weyl algebras rather than skew polynomial rings.

An ideal \(\mathfrak {p}\) of a ring *R* is called a *completely prime ideal* if the factor ring \(R/\mathfrak {p}\) is a domain. A completely prime ideal is a prime ideal. The sets of prime and completely prime ideals of the ring *R* are denoted by \(\mathrm{Spec}(R)\) and \(\mathrm{Spec}_c(R)\), respectively.

In Theorem 1.1, a classification of prime and completely prime ideals of the algebra \(\Lambda \) is given, the Krull and global dimensions of the algebra \(\Lambda \) are found. The algebra \(\Lambda \) is a Noetherian domain of Gelfand-Kirillov dimension 2.

### Theorem 1.1

*K*be a field of characteristic zero, \(\Lambda =K[X][Y; \delta := f\frac{d}{dX} ]\) where \(f\in K[X]\backslash K\). Let \(f=p_1^{n_1}\cdots p_s^{n_s}\) be a unique (up to permutation) product of irreducible polynomials of

*K*[

*X*]. Then

- 1.
The Krull dimension of \(\Lambda \) is \(\mathrm{Kdim } (\Lambda )=2\).

- 2.
The global dimension of \(\Lambda \) is \(\mathrm{gldim}(\Lambda )=2\).

- 3.
The elements \(p_1, \ldots , p_s\) are regular normal elements of the algebra \(\Lambda \) (i.e. \(p_i\) is a non-zero-divisor of \(\Lambda \) and \(p_i\Lambda = \Lambda p_i\)).

- 4.
\(\mathrm{Spec}(\Lambda )=\mathrm{Spec}_c (\Lambda )=\{ 0 , \Lambda p_i, (p_i, q_i)\, | \, i=1, \ldots , s;\; q_i\in \mathrm{Irr}_m(F_i[Y])\}\) where \(F_i:=K[X]/(p_i)\) is a field and \(\mathrm{Irr}_m(F_i[Y])\) is the set of monic irreducible polynomials of the polynomial algebra \(F_i[Y]\) over the field \(F_i\) in the variable

*Y*. If, in addition, the field*K*is an algebraically closed and \(\lambda _1, \ldots , \lambda _s\) are the roots of the polynomial*f*then \(\mathrm{Spec}(\Lambda )=\{ 0 , \Lambda (X-\lambda _i), (X-\lambda _i, Y-\mu )\, | \, i=1, \ldots , s;\; \mu \in K\}\).

## 2 Classification of Simple \(\Lambda \)-Modules

In this section, ‘module’ means left module, *K* is an algebraically closed field of characteristic zero, \(\Lambda =K[X][Y, \delta =f\frac{d}{dX}]\) where \(f\in K[X]\backslash K\). The algebra \(\Lambda \) is a Noetherian domain. The aim of the section is to give a classification of simple \(\Lambda \)-modules (Lemma 2.1 and Theorem 2.10).

**The element**

*f*

**is a regular normal element of**\(\Lambda \). It follows from

*f*is a

*normal*element of \(\Lambda \) (i.e. \(\Lambda f=f\Lambda )\). It determines a

*K*-automorphism \(\omega _f\) of the algebra \(\Lambda \):

**The Weyl algebra**\(A_1\)

**is a generalized Weyl algebra.**The Weyl algebra \(A_1\) is a simple Noetherian domain with

*restricted minimum condition*, i.e. any proper left or right factor module of \(A_1\) has finite length, [10].

*Definition,*[1, 2]. Let

*D*be a ring, \(\sigma \) be an automorphisms of

*D*and

*a*be a central element of

*D*. A

**generalized Weyl algebra**(GWA) \(A=D(\sigma , a)\) of degree 1, is the ring generated by

*D*and by two indeterminates

*X*and

*Y*subject to the relations [1, 2]: For all \(\alpha \in D\),

*K*[

*H*] where \(\sigma \in \mathrm{Aut}_K\,K[H]\) and \(\sigma :H\rightarrow H-1\).

*f*, i.e.

*f*-

*torsion*\((M_f=0)\) and

*f*-

*torsionfree*\((M_f\ne 0)\) simple \(\Lambda \)-modules and \(A_1\)-modules, respectively,

*f*-torsion \(\Lambda \)-modules.

### Lemma 2.1

*f*. Then

### Proof

*f*-torsion \(\Lambda \)-module

*M*is annihilated by the (normal) element

*f*\((fM=0)\). So, in fact, the \(\Lambda \)-module

*M*is a simple module over the factor algebra

*K*[

*X*,

*Y*] in two variables at the ideal (

*f*) generated by

*f*, and the equality in the lemma follows.

*N*be a simple \(\Lambda \)-module. Then the map

*f*is normal in \(\Lambda )\). In the second case

*N*is, in fact, a simple \((\Lambda _f\equiv A_{1,f})\)-module, so \(\mathrm{dim }_K\,N=\infty \), since \(A_{1,f}\) is a simple infinite dimensional algebra. If the module

*N*is finite dimensional, then \(fN=0\), i.e. \([N]\in \hat{\Lambda }(f-\mathrm{torsion})\). \(\square \)

\(\hat{\Lambda }(f-\mathrm{torsionfree})\). The sum of all simple submodules of a \(\Lambda \)-module *M* is called the *socle* of *M* which is denoted by \(\mathrm{soc}_\Lambda M \). It is the largest semisimple submodule of *M*. A \(\Lambda \)-module *N* is called \(\Lambda \)-*socle* (or, *socle*, for short) provided \(\mathrm{soc}_\Lambda N\ne 0\). Denote by \(\hat{A_1}(\Lambda \)-socle) the set of isoclasses of simple \(\Lambda \)-socle \(A_1\)-modules. The proof of the following lemma is evident (see [2, Lemma 3.4] for details).

### Lemma 2.2

- 1.The canonical mapis a bijection with inverse \([N]\rightarrow [\mathrm{soc}_\Lambda (N)]\).$$\begin{aligned} (\cdot )_f :\hat{\Lambda } (f-\mathrm{torsionfree})\rightarrow \hat{A}_{1,f} (\Lambda -\mathrm{socle}),\;[M]\mapsto [ M_f:=A_{1,f}\otimes _\Lambda M] \end{aligned}$$
- 2.Each simple
*f*-torsionfree \(\Lambda \)-module has the formfor some maximal left ideal \(\mathfrak {m}\) of the ring \(A_{1,f} \). Two such modules are isomorphic, \(M_\mathfrak {m}\simeq M_\mathfrak {n}\), iff the \(A_{1,f}\)-modules \(A_{1,f}/\mathfrak {m}\) and \(A_{1,f}/\mathfrak {n}\) are isomorphic. \(\square \)$$\begin{aligned} M_\mathfrak {m}:= \Lambda /\Lambda \cap \mathfrak {m}\end{aligned}$$(6)

### Lemma 2.3

*f*. Then

### Proof

*K*[

*y*] in a variable \(y=\partial +A_1(X-\lambda _i)\) and

*y*. Thus the linear operator

Now, let \([M]\in \hat{A}_1(f-\mathrm{torsion})\). Then there exists *i* such that *M* is an epimorphic image of \(M_i\), hence \(M\simeq M_i\).

\(\square \)

### Theorem 2.4

### Proof

The map above is well defined and injective.

Let \([N]\in \hat{A}_{1,f}\). Then \(N\simeq A_{1,f}/J\) for some nonzero maximal left ideal *J* of \(A_{1,f}\). Then \(I=J\cap A_1\ne 0\) and \(A_1/I\) is a \(A_1\)-submodule of *N*. The \(A_1\)-module \(A_1/I\) has finite length [10], thus it contains a simple \(A_1\)-submodule, say *M*. Then \(N\simeq M_f\) which means that the map above is surjective.\(\square \)

*skew Laurent polynomial ring*

*K*(

*H*) of rational functions. The algebra

*B*is a right and left

*Euclidean*domain with respect to the ‘length’ function

*M*belongs to the first (resp., second) set if \(S^{-1}M=0\) (resp., \(S^{-1}M\ne 0\)).

For \(\lambda \in K\) set \(\mathcal{O}(\lambda ):=\lambda +\mathbb {Z}\). We say that scalars \(\lambda \) and \(\mu \) are *equivalent*, \(\lambda \sim \mu \), if either \(\mathcal{O}(\lambda )=\mathcal{O}(\mu )\ne \mathbb {Z}\) or both \(\lambda \) and \(\mu \) belong either to \((-\infty ,0]:=\{ i\in \mathbb {Z}\, | \, i\le 0\}\) or to \([1,\infty ):=\{ i\in \mathbb {Z}\, | \, i\ge 1\}\). Then \(\sim \) is an equivalence relation on *K*. Let \(K/\sim \) be the set of equivalence classes of *K* under \(\sim \). So, the elements of the set \(K/\sim \) are distinct sets \(\lambda +\mathbb {Z}\) where \(\lambda \not \in \mathbb {Z}\) and the two sets \((-\infty ,0]\) and \([1,\infty )\). Notice that \(\mathbb {Z}= (-\infty ,0]\coprod [1,\infty )\).

### Proposition 2.5

- 1.
If \(\Gamma =\mathcal{O}(\lambda )\ne \mathbb {Z}\), then \(L(\Gamma )= A_1/A_1(H-\lambda )\).

- 2.
If \(\Gamma =(-\infty ,0]\), then \(L(\Gamma )= A_1/A_1X\).

- 3.
If \(\Gamma =[1,\infty )\), then \(L(\Gamma )= A_1/A_1(H-1, Y)\). \(\Box \)

### Corollary 2.6

### Proof

Straightforward. \(\square \)

### Corollary 2.7

- 1.
If \(M=A_1/A_1X\) (i.e. 0 is not a root of

*f*, by Corollary 2.6) then*M*is a simple*f*-torsionfree \(\Lambda \)-module with \(M=M_f\). - 2.
If \(M\ne A_1/A_1X\) then \(\mathrm{soc}_\Lambda \, M=\mathrm{soc}_\Lambda \, M_f=0\). The set \(\hat{\Lambda }\) (\(D-\)torsion, \(f-\)torsionfree) is equal to \(\{A_1/A_1/X\}\) if 0 is not a root of

*f*and \(\emptyset \), otherwise.

### Proof

*M*is a simple

*f*-torsionfree \(\Lambda \)-module \((f(0)\ne 0)\). Moreover, the linear map \(f_M:M\rightarrow M\), \(m\mapsto fm\) is a bijection, hence, \(M=M_f\).

*M*belongs to the first (resp., third) class of modules from Proposition 2.5 , i.e.

*M*. In both cases, for \(i\ge 0\), set \(x^i=X^i\bar{1}\). In the first case, for \(i<0\), set \(x^i=\mu _i\partial ^{-i}\bar{1}\), \(\mu _i\in K\). The scalars \(\mu _i\) can be chosen in such a way that (in both cases) \( Xx^i=x^{i+1}\) for all possible

*i*. Degree argument shows that the module

*M*contains a strictly descending chain of \(\Lambda \)-submodules

*N*is an essential simple \(\Lambda \)-submodule of both \(M_f\) and

*M*, hence \(0\ne N\subseteq \cap _{n\ge 0}\; f^nM=0\), a contradiction. \(\square \)

An element of a ring is called *regular* if it is not a zero divisor. Given a ring *A* and a multiplicatively closed subset *S* of *A* which consists of regular normal elements. Let \(B=S^{-1} A\) be the localization of *A* at *S*.

### Theorem 2.8

*A*,

*B*, and

*S*be as above and let \(\mathfrak {m}\) be a maximal left ideal of

*B*. The following are equivalent.

- 1.
The

*A*-module \(M_\mathfrak {m}:=A/A\cap \mathfrak {m}\) is simple. - 2.
The socle \(\mathrm{soc}_A(M_\mathfrak {m})\ne 0\).

- 3.
\(A=As+A\cap \mathfrak {m}\) for all \(s\in S\). \(\square \)

*Remark*. If \(S=\{ f^n, n\ge 0\}\) for some regular normal element *f* of *A*, then the last condition of this lemma is equivalent to \(A=Af+A\cap \mathfrak {m}\). We shall use this fact in what follows. In general situation, it suffices to check whether the third condition holds only for generators of the monoid *S*.

### Proof

The implications \((1\Rightarrow 2)\) and \((1\Rightarrow 3)\) are obvious.

*A*-module which for some \(s\in S\) is equal to

*A*-module \(M_{\mathfrak {m}} \) by the automorphism \(\omega _s^{-1}\) of

*A*(the element

*s*is regular and normal). Since the

*A*-module \({}^{\omega _s^{-1} }M_{\mathfrak {m}} \) is simple, so is \(M_{\mathfrak {m}}\).

\((3\Rightarrow 1)\) If *J* is a left ideal of *A* which contains \(A\cap \mathfrak {m}\) but does not coincide with it, then, by the maximality of \(\mathfrak {m}\), \(S^{-1} J=B\). Therefore \(J\cap S\ne \emptyset \). Let \(s\in J\cap S\). Then \(J\supseteq As+A\cap \mathfrak {m}=A\), that is \(M_\mathfrak {m}\) is a simple *A*-module.

\(\square \)

\(\hat{A}_1(D-\mathrm{torsionfree})\). Let us recall a description of \(\hat{A}_1(D-\mathrm{torsionfree})\) from [2]. In the set \(S=K[H]\backslash \{ 0\}\) consider the relation <: \(\alpha <\beta \) if there are no roots \(\lambda \) and \(\mu \) of the polynomial \(\alpha \) and \(\beta \) respectively and such that \(\lambda -\mu \) is non-negative integer.

*Definition,* [2]. An element \(b=Y^m\beta _{-m}+\cdots +\beta _0\in A_1\), \(m>0\), all \(\beta _i\in D\), is called **l-normal** if \(\beta _0<\beta _{-m}\) and \(\beta _0<H\), (i.e. the polynomial \(\beta _0\) has no root from \(\{ 0, 1, 2, \ldots \}\) and there are no roots \(\lambda \) and \(\mu \) of the polynomials \(\beta _0\) and \(\beta _m \) respectively with \(\lambda -\mu \in \{ 0, 1, 2, \ldots \}\)).

### Theorem 2.9

*B*. Then

*D*-torsionfree \(A_1\)-module. Two such \(A_1\)-modules are isomorphic, \( \mathcal{M}_b\simeq \mathcal{M}_c\), iff \(B/Bb\simeq B/Bc\) as

*B*-modules. Each simple

*D*-torsionfree \(A_1\)-module is isomorphic to some \( \mathcal{M}_b\). \(\square \)

*localization of the (left)*\(\Lambda \)

*-module*

*B*at \(S_f\). Then the algebra \(A_{1,f}= \Lambda _f\) can be considered as a (\(A_{1,f}= \Lambda _f\))-submodules of \(B_f\). For any nonzero \(b\in B\), \((Bb)_f=B_fb\).

Theorem 2.10 is a classification of simple *f*-torsionfree \(\Lambda \)-modules.

### Theorem 2.10

*B*such that

- 1.
\(\Lambda =\Lambda f+\Lambda \cap B_fb \; (= \Lambda f+\Lambda \cap Bb )\), and

- 2.
the simple

*B*-module*B*/*Bb*is not isomorphic to any of modules \(B/B(X-\lambda )\) where \(\lambda \) runs through the nonzero roots of*f*.

*f*-torsionfree \(\Lambda \)-module. Two such \(\Lambda \)-modules are isomorphic, \( \mathcal {M}_b\simeq \mathcal {M}_c\), iff \(B/Bb\simeq B/Bc\) as

*B*-modules.

Each simple *f*-torsionfree \(\Lambda \)-module is isomorphic either to some \( \mathcal {M}_b\) or to the module \(M=A_1/A_1X\) from Corollary 2.7, if 0 is not a root of *f* (the \(\Lambda \)-module *M* is not isomorphic to any \(\mathcal {M}_b\)). The condition 1 above is equivalent to the condition that \(\Lambda = \Lambda (X-\lambda _i)+\Lambda \cap B_fb \; (= \Lambda (X-\lambda _i)+\Lambda \cap Bb )\) for all roots \(\lambda _i\) of the polynomial *f*.

Each simple *f*-torsionfree \(\Lambda \)-module is infinite dimensional.

### Proof

*f*) or \(M_f\in \hat{A}_{1,f}(D-\mathrm{torsionfree}, \Lambda -\mathrm{socle})\). In the first case, \(M=\mathrm{soc}_\Lambda (M_f)=M_f=A_1/A_1X\) (Corollary 2.7).

*b*from Theorem 2.9. Note that the left ideal \(\mathfrak {m}=A_{1,f}\cap B_fb\) of \(A_{1,f}\) is maximal. By Lemma 2.3 and Theorem 2.9, \([\mathcal{M}_b]\in \hat{A}_1(D-\mathrm{torsionfree}, f-\mathrm{torsionfree})\) iff the second condition of the theorem holds. Now,

*f*-torsionfree \(\Lambda \)-submodule of \(M_f\). If \(\mathrm{ker}\, \varphi \ne 0\), then \(\mathrm{soc}_\Lambda (M_f)\subseteq \mathrm{ker}\, \varphi \), but \(\mathrm{ker}\, \varphi \) is an

*f*-torsion \(\Lambda \)-module, a contradiction.

*B*-modules.

The condition 1 of the theorem is equivalent to the condition that \(\Lambda = \Lambda (X-\lambda _i)+\Lambda \cap B_fb \; (= \Lambda (X-\lambda _i)+\Lambda \cap Bb )\) for all roots \(\lambda _i\) of the polynomial *f* (since the elements \(X-\lambda _i\) are regular normal elements of \(\Lambda \) and \(\lambda _i\) are the roots of *f*).

By Lemma 2.1, each simple *f*-torsionfree \(\Lambda \)-module is infinite dimensional. If 0 is not a root of *f*, then the modules \(M=A_1/A_1X\) and \(\mathcal{M}_b\) (from the theorem) are not isomorphic, since the linear map \(X_M: M\rightarrow M\), \(m\mapsto Xm\) is locally nilpotent but \(\mathrm{ker}\, X_{\mathcal{M}_b}=0\). \(\square \)

## 3 The Prime Ideals, the Krull and Global Dimensions of the Algebra \(\Lambda \)

In this section, *K* is a field of characteristic zero (not necessarily algebraically closed) and \(f=p_1^{n_1}\cdots p_s^{n_s}\) is a nonscalar polynomial of *K*[*X*] where \(p_1, \ldots , p_s\) are irreducible, co-prime divisors of *f* (i.e. \(K[X]p_i+K[X]p_j=K[X]\) for all \(i\ne j\)). The aim of this section is to give a proof of Theorem 1.1.

### Proof of Theorem 1.1

By (3), \(\Lambda _f=A_{1,f}\) is a simple algebra (as a localization of a simple Noetherian algebra). If \(\mathfrak {p}\) is a nonzero prime ideal of the algebra \(\Lambda \) then \(f^n\in \mathfrak {p}\) for some natural number \(n\ge 1\). Hence, \(p_i\in \mathfrak {p}\) for some *i*, by statement 3. By (8), \(\mathfrak {p}= (p_i, g_i)\) for some monic irreducible polynomial \(g_i\) of the polynomial algebra \(F_i[Y]\).

1. By [11, Theorem 6.5.4.(i)], \(\mathrm{Kdim } (\Lambda ) \le \mathrm{Kdim } (K[X])+1=1+1=2\).

2. By [11, Theorem 7.5.3.(i)], \(\mathrm{gldim}(\Lambda ) \le \mathrm{gldim}(K[X])+1=1+1=2\).

## Notes

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