Left A-Modules, Right A-Modules, and A-Bimodules

# Left A-Modules, Right A-Modules, and A-Bimodules

## Left A-Modules, Right A-Modules, and A-Bimodules

 Definition: Let $\mathfrak{A}$ be an algebra over $\mathbf{F}$ and let $M$ be a linear space over $\mathbf{F}$. We say that $M$ is a Left $\mathfrak{A}$-Module if with the mapping of left module multiplication $(x, m) \to xm$ of $\mathfrak{A} \times M$ into $M$ is such that the following axioms are satisfied: LM 1) For each fixed $a \in \mathfrak{A}$ the map $m \to am$ is linear on $M$. LM 2) For each fixed $m \in M$ the map $a \to am$ is linear on $\mathfrak{A}$. LM 3) For all $a_1, a_2 \in \mathfrak{A}$ and all $m \in M$ we have that $a_1(a_2m) = (a_1a_2)m$. Similarly, we say that $M$ is a Right $\mathfrak{A}$-Module if with the mapping of right module multiplication $(a, m) \to ma$ of $\mathfrak{A} \times M$ into $M$ is such that the following axioms are satisfied: RM 1) For each fixed $a \in \mathfrak{A}$ the map $m \to ma$ is linear on $M$. RM 2) For each fixed $m \in M$ the map $a \to ma$ is linear on $\mathfrak{A}$. RM 3) For all $a_1, a_2 \in \mathfrak{A}$ and all $m \in M$ we have that $(ma_1)a_2 = m(a_1a_2)$. We say that $M$ is an $\mathfrak{A}$-Bimodule if it is both a left $\mathfrak{A}$-module and a right $\mathfrak{A}$-module, as well as the additional axiom holds: BM 1) For all $a_1, a_2 \in \mathfrak{A}$ and all $m \in M$ we have that $a_1(ma_2) = (a_1m)a_2$.
 Definition: Let $\mathfrak{A}$ be an algebra with unit $e$. 1) If $M$ is a left $\mathfrak{A}$-module then $M$ is said to be Unit Linked Left $\mathfrak{A}$-Module if for every $m \in M$ we have that $em = m$. 2) If $M$ is a right $\mathfrak{A}$-module then $M$ is said to be Unit Linked Right $\mathfrak{A}$-Module if for every $m \in M$ we have that $me = m$. 3) If $M$ is an $\mathfrak{A}$-bimodule then $M$ is said to be Unit Linked $\mathfrak{A}$-Bimodule if for every $m \in M$ we have that $em = me = m$.

### Example 1

Let $\mathfrak{A} = \mathbb{R}$ and let $M$ be any linear space over $\mathbb{R}$. Then $M$ is an $\mathbb{R}$-bimodule with module multiplication $f : \mathbb{R} \times M \to M$ defined for all $(t, m) \in \mathbb{R} \times M$ by $f(t, m) = tm$

Clearly for each fixed $t \in \mathbb{R}$ the map $f_t : M \to M$ defined by $f_t(m) = tm$ is linear since $t(m_1 + m_2) = tm_1 + tm_2$ for all $m_1, m_2 \in M$. Also it is clear that for each fixed $m \in M$ the map $f_m : \mathbb{R} \to M$ defined by $f_m(t) = tm$ is linear since $(t_1 + t_2)m = t_1m + t_2m$ for all $t_1, t_2 \in \mathbb{R}$. Lastly, we have that $s(tm) = (st)m$ for all $s, t \in \mathbb{R}$ and $m \in M$.

So every linear space $M$ over $\mathbb{R}$ is a left $\mathbb{R}$-module. It is easy to check that every linear space $M$ is also a right $\mathbb{R}$-module and that it is actually an $\mathbb{R}$-bimodule that is unit-linked by the unit number 1 in $\mathbb{R}$.

### Example 2

Let:

(1)
\begin{align} \quad \mathfrak{A} = \left \{ \begin{bmatrix} x_{11} & x_{12} \\ x_{21} & x_{22} \end{bmatrix} : x_{11}, x_{12}, x_{21}, x_{22} \in \mathbb{R} \right \} \end{align}

That is, $\mathfrak{A}$ consists of all $2 \times 2$ matrices with real entries. With the operations of matrix addition and scalar multiplication, and matrix multiplication, it is clear that $\mathfrak{A}$ is an algebra over $\mathbb{R}$. Let $M = \mathbb{R}^2$. Then $M$ can be viewed as a left $\mathfrak{A}$-module with module multiplication $\mathfrak{A} \times M \to M$ by:

(2)
\begin{align} \quad \begin{bmatrix} x_{11} & x_{12} \\ x_{21} & x_{22} \end{bmatrix} (a, b) = (x_{11}a + x_{12}b, x_{21}a + x_{22}b) \end{align}

It is also unit linked where the unit in $\mathfrak{A}$ is the $2 \times 2$ identity matrix $I_2$ since clearly for any $(a, b) \in \mathbb{R}^2$ we have that $I_2(a, b) = (a, b)$.

## Banach Left A-Modules, Banach Right A-Modules, and Banach A-Bimodules

 Definition: Let $\mathfrak{A}$ be a normed algebra and let $M$ be a normed linear space, both over $\mathbf{F}$. 1) $M$ is said to be a Normed Left $\mathfrak{A}$-Module if $M$ is a left $\mathfrak{A}$-module and there exists a $K > 0$ such that for all $a \in \mathfrak{A}$ and all $m \in M$ we have that $\| am \|_M \leq K \| a \|_{\mathfrak{A}} \| m \|_M$. 2) $M$ is said to be a Normed Right $\mathfrak{A}$-Module if $M$ is a right $\mathfrak{A}$-module and there exists a $K > 0$ such that for all $a \in \mathfrak{A}$ and all $m \in M$ we have that $\| ma \|_M \leq K \| a \|_{\mathfrak{A}} \| m \|_M$. 3) $M$ is said to be a Normed $\mathfrak{A}$-Bimodule if $M$ is both a normed left $\mathfrak{A}$-module and a normed right $\mathfrak{A}$-module. A normed right/left/ $\mathfrak{A}$-(bi)module is said to be a Banach Left/Right $\mathfrak{A}$-(Bi)Module if $M$ is also a Banach space.