# Graph C*-algebra

In mathematics, a graph C*-algebra is a universal C*-algebra constructed from a directed graph. Graph C*-algebras are direct generalizations of the Cuntz algebras and Cuntz-Krieger algebras, but the class of graph C*-algebras has been shown to also include several other widely studied classes of C*-algebras. As a result, graph C*-algebras provide a common framework for investigating many well-known classes of C*-algebras that were previously studied independently. Among other benefits, this provides a context in which one can formulate theorems that apply simultaneously to all of these subclasses and contain specific results for each subclass as special cases.

Although graph C*-algebras include numerous examples, they provide a class of C*-algebras that are surprisingly amenable to study and much more manageable than general C*-algebras. The graph not only determines the associated C*-algebra by specifying relations for generators, it also provides a useful tool for describing and visualizing properties of the C*-algebra. This visual quality has led to graph C*-algebras being referred to as "operator algebras we can see."[1][2] Another advantage of graph C*-algebras is that much of their structure and many of their invariants can be readily computed. Using data coming from the graph, one can determine whether the associated C*-algebra has particular properties, describe the lattice of ideals, and compute K-theoretic invariants.

## Graph terminology

The terminology for graphs used by C*-algebraists differs slightly from that used by graph theorists. The term graph is typically taken to mean a directed graph ${\displaystyle E=(E^{0},E^{1},r,s)}$ consisting of a countable set of vertices ${\displaystyle E^{0}}$, a countable set of edges ${\displaystyle E^{1}}$, and maps ${\displaystyle r,s:E^{1}\rightarrow E^{0}}$ identifying the range and source of each edge, respectively. A vertex ${\displaystyle v\in E^{0}}$ is called a sink when ${\displaystyle s^{-1}(v)=\emptyset }$; i.e., there are no edges in ${\displaystyle E}$ with source ${\displaystyle v}$. A vertex ${\displaystyle v\in E^{0}}$ is called an infinite emitter when ${\displaystyle s^{-1}(v)}$ is infinite; i.e., there are infinitely many edges in ${\displaystyle E}$ with source ${\displaystyle v}$. A vertex is called a singular vertex if it is either a sink or an infinite emitter, and a vertex is called a regular vertex if it is not a singular vertex. Note that a vertex ${\displaystyle v}$ is regular if and only if the number of edges in ${\displaystyle E}$ with source ${\displaystyle v}$ is finite and nonzero. A graph is called row-finite if it has no infinite emitters; i.e., if every vertex is either a regular vertex or a sink.

A path is a finite sequence of edges ${\displaystyle e_{1}e_{2}\ldots e_{n}}$ with ${\displaystyle r(e_{i})=s(e_{i+1})}$ for all ${\displaystyle 1\leq i\leq n-1}$. An infinite path is a countably infinite sequence of edges ${\displaystyle e_{1}e_{2}\ldots }$ with ${\displaystyle r(e_{i})=s(e_{i+1})}$ for all ${\displaystyle i\in \mathbb {N} }$. A cycle is a path ${\displaystyle e_{1}e_{2}\ldots e_{n}}$ with ${\displaystyle r(e_{n})=s(e_{1})}$, and an exit for a cycle ${\displaystyle e_{1}e_{2}\ldots e_{n}}$ is an edge ${\displaystyle f\in E^{1}}$ such that ${\displaystyle s(f)=s(e_{i})}$ and ${\displaystyle f\neq e_{i}}$ for some ${\displaystyle 1\leq i\leq n}$. A cycle ${\displaystyle e_{1}e_{2}\ldots e_{n}}$ is called a simple cycle if ${\displaystyle s(e_{i})\neq s(e_{1})}$ for all ${\displaystyle 2\leq i\leq n}$.

The following are two important graph conditions that arise in the study of graph C*-algebras.

Condition (L): Every cycle in the graph has an exit.

Condition (K): There is no vertex in the graph that is on exactly one simple cycle. Equivalently, a graph satisfies Condition (K) if and only if each vertex in the graph is either on no cycles or on two or more simple cycles.

## The Cuntz-Krieger Relations and the Universal Property

A Cuntz-Krieger ${\displaystyle E}$-family is a collection ${\displaystyle \{s_{e},p_{v}:e\in E^{1},v\in E^{0}\}}$ in a C*-algebra such that the elements of ${\displaystyle \{s_{e}:e\in E^{1}\}}$ are partial isometries with mutually orthogonal ranges, the elements of ${\displaystyle \{p_{v}:v\in E^{0}\}}$ are mutually orthogonal projections, and the following three relations (called the Cuntz-Krieger relations) are satisfied:

(CK1) ${\displaystyle s_{e}^{*}s_{e}=p_{r(e)}}$ for all ${\displaystyle e\in E^{1}}$,

(CK2) ${\displaystyle p_{v}=\sum _{s(e)=v}s_{e}s_{e}^{*}}$ whenever ${\displaystyle v}$ is a regular vertex, and

(CK3) ${\displaystyle s_{e}s_{e}^{*}\leq p_{s(e)}}$ for all ${\displaystyle e\in E^{1}}$.

The graph C*-algebra corresponding to ${\displaystyle E}$, denoted by ${\displaystyle C^{*}(E)}$, is defined to be the C*-algebra generated by a Cuntz-Krieger ${\displaystyle E}$-family that is universal in the sense that whenever ${\displaystyle \{t_{e},q_{v}:e\in E^{1},v\in E^{0}\}}$ is a Cuntz-Krieger ${\displaystyle E}$-family in a C*-algebra ${\displaystyle A}$ there exists a ${\displaystyle *}$-homomorphism ${\displaystyle \phi :C^{*}(E)\to A}$ with ${\displaystyle \phi (s_{e})=t_{e}}$ for all ${\displaystyle e\in E^{1}}$ and ${\displaystyle \phi (p_{v})=q_{v}}$ for all ${\displaystyle v\in E^{0}}$. Existence of ${\displaystyle C^{*}(E)}$ for any graph ${\displaystyle E}$ was established by Kumjian, Pask, and Raeburn.[3] Uniqueness of ${\displaystyle C^{*}(E)}$ (up to ${\displaystyle *}$-isomorphism) follows directly from the universal property.

### Edge Direction Convention

It is important to be aware that there are competing conventions regarding the "direction of the edges" in the Cuntz-Krieger relations. Throughout this article, and in the way that the relations are stated above, we use the convention first established in the seminal papers on graph C*-algebras.[3][4] The alternate convention, which is used in Raeburn's CBMS book on Graph Algebras,[5] interchanges the roles of the range map ${\displaystyle r}$ and the source map ${\displaystyle s}$ in the Cuntz-Krieger relations. The effect of this change is that the C*-algebra of a graph for one convention is equal to the C*-algebra of the graph with the edges reversed when using the other convention.

### Row-Finite Graphs

In the Cuntz-Krieger relations, (CK2) is imposed only on regular vertices. Moreover, if ${\displaystyle v\in E^{0}}$ is a regular vertex, then (CK2) implies that (CK3) holds at ${\displaystyle v}$. Furthermore, if ${\displaystyle v\in E^{0}}$ is a sink, then (CK3) vacuously holds at ${\displaystyle v}$. Thus, if ${\displaystyle E}$ is a row-finite graph, the relation (CK3) is superfluous and a collection ${\displaystyle \{s_{e},p_{v}:e\in E^{1},v\in E^{0}\}}$ of partial isometries with mutually orthogonal ranges and mutually orthogonal projections is a Cuntz-Krieger ${\displaystyle E}$-family if and only if the relation in (CK1) holds at all edges in ${\displaystyle E}$ and the relation in (CK2) holds at all vertices in ${\displaystyle E}$ that are not sinks. The fact that the Cuntz-Krieger relations take a simpler form for row-finite graphs has technical consequences for many results in the subject. Not only are results easier to prove in the row-finite case, but also the statements of theorems are simplified when describing C*-algebras of row-finite graphs. Historically, much of the early work on graph C*-algebras was done exclusively in the row-finite case. Even in modern work, where infinite emitters are allowed and C*-algebras of general graphs are considered, it is common to state the row-finite case of a theorem separately or as a corollary, since results are often more intuitive and transparent in this situation.

## Examples

The graph C*-algebra has been computed for many graphs. Conversely, for certain classes of C*-algebras it has been shown how to construct a graph whose C*-algebra is ${\displaystyle *}$-isomorphic or Morita equivalent to a given C*-algebra of that class.

The following table shows a number of directed graphs and their C*-algebras. We use the convention that a double arrow drawn from one vertex to another and labeled ${\displaystyle \infty }$ indicates that there are a countably infinite number of edges from the first vertex to the second.

Directed Graph ${\displaystyle E}$ Graph C*-algebra ${\displaystyle C^{*}(E)}$
${\displaystyle \mathbb {C} }$, the complex numbers
${\displaystyle C(\mathbb {T} )}$, the complex-valued continuous functions on the circle ${\displaystyle \mathbb {T} }$
${\displaystyle M_{n}(\mathbb {C} )}$, the ${\displaystyle n\times n}$ matrices with entries in ${\displaystyle \mathbb {C} }$
${\displaystyle {\mathcal {K}}}$, the compact operators on a separable infinite-diemnsional Hilbert space
${\displaystyle M_{n}(C(\mathbb {T} ))}$, the ${\displaystyle n\times n}$ matrices with entries in ${\displaystyle C(\mathbb {T} )}$
${\displaystyle {\mathcal {O}}_{n}}$, the Cuntz algebra generated by ${\displaystyle n}$ isometries
${\displaystyle {\mathcal {O}}_{\infty }}$, the Cuntz algebra generated by a countably infinite number of isometries
${\displaystyle {\mathcal {K}}^{1}}$, the unitization of the algebra of compact operators ${\displaystyle {\mathcal {K}}}$
${\displaystyle {\mathcal {T}}}$, the Toeplitz algebra

The class of graph C*-algebras has been shown to contain various classes of C*-algebras. The C*-algebras in each of the following classes may be realized as graph C*-algebras up to ${\displaystyle *}$-isomorphism:

The C*-algebras in each of the following classes may be realized as graph C*-algebras up to Morita equivalence:

• AF algebras[6]
• Kirchberg algebras with free K1-group

## Correspondence between graph and C*-algebraic properties

One remarkable aspect of graph C*-algebras is that the graph ${\displaystyle E}$ not only describes the relations for the generators of ${\displaystyle C^{*}(E)}$, but also various graph-theoretic properties of ${\displaystyle E}$ can be shown to be equivalent to C*-algebraic properties of ${\displaystyle C^{*}(E)}$. Indeed, much of the study of graph C*-algebras is concerned with developing a lexicon for the correspondence between these properties, and establishing theorems of the form "The graph ${\displaystyle E}$ has a certain graph-theoretic property if and only if the C*-algebra ${\displaystyle C^{*}(E)}$ has a corresponding C*-algebraic property." The following table provides a short list of some of the more well-known equivalences.

Property of ${\displaystyle E}$ Property of ${\displaystyle C^{*}(E)}$
${\displaystyle E}$ is a finite graph. ${\displaystyle C^{*}(E)}$ is finite dimensional.
The vertex set ${\displaystyle E^{0}}$ is finite. ${\displaystyle C^{*}(E)}$ is unital (i.e., ${\displaystyle C^{*}(E)}$ contains a multiplicative identity).
${\displaystyle E}$ has no cycles. ${\displaystyle C^{*}(E)}$ is an AF algebra.
${\displaystyle E}$ satisfies the following three properties:
1. Condition (L),
2. for each vertex ${\displaystyle v}$ and each infinite path ${\displaystyle \alpha }$ there exists a directed path from ${\displaystyle v}$ to a vertex on ${\displaystyle \alpha }$, and
3. for each vertex ${\displaystyle v}$ and each singular vertex ${\displaystyle w}$ there exists a directed path from ${\displaystyle v}$ to ${\displaystyle w}$
${\displaystyle C^{*}(E)}$ is simple.
${\displaystyle E}$ satisfies the following three properties:
1. Condition (L),
2. for each vertex ${\displaystyle v}$ in ${\displaystyle E}$ there is a path from ${\displaystyle v}$ to a cycle.
Every hereditary subalgebra of ${\displaystyle C^{*}(E)}$ contains an infinite projection.
(When ${\displaystyle C^{*}(E)}$ is simple this is equivalent to ${\displaystyle C^{*}(E)}$ being purely infinite.)

## The gauge action

The universal property produces a natural action of the circle group ${\displaystyle \mathbb {T} :=\{z\in \mathbb {C} :|z|=1\}}$ on ${\displaystyle C^{*}(E)}$ as follows: If ${\displaystyle \{s_{e},p_{v}:e\in E^{1},v\in E^{0}\}}$ is a universal Cuntz-Krieger ${\displaystyle E}$-family, then for any unimodular complex number ${\displaystyle z\in \mathbb {T} }$, the collection ${\displaystyle \{zs_{e},p_{v}:e\in E^{1},v\in E^{0}\}}$ is a Cuntz-Krieger ${\displaystyle E}$-family, and the universal property of ${\displaystyle C^{*}(E)}$ implies there exists a ${\displaystyle *}$-homomorphism ${\displaystyle \gamma _{z}:C^{*}(E)\to C^{*}(E)}$ with ${\displaystyle \gamma _{z}(s_{e})=zs_{e}}$ for all ${\displaystyle e\in E^{1}}$ and ${\displaystyle \gamma _{z}(p_{v})=p_{z}}$ for all ${\displaystyle v\in E^{0}}$. For each ${\displaystyle z\in \mathbb {T} }$ the ${\displaystyle *}$-homomorphism ${\displaystyle \gamma _{\overline {z}}}$ is an inverse for ${\displaystyle \gamma _{z}}$, and thus ${\displaystyle \gamma _{z}}$ is an automorphism. This yields a strongly continuous action ${\displaystyle \gamma :\mathbb {T} \to \operatorname {Aut} C^{*}(E)}$ by defining ${\displaystyle \gamma (z):=\gamma _{z}}$. The gauge action ${\displaystyle \gamma }$ is sometimes called the canonical gauge action on ${\displaystyle C^{*}(E)}$. It is important to note that the canonical gauge action depends on the choice of the generating Cuntz-Krieger ${\displaystyle E}$-family ${\displaystyle \{s_{e},p_{v}:e\in E^{1},v\in E^{0}\}}$. The canonical gauge action is a fundamental tool in the study of ${\displaystyle C^{*}(E)}$. It appears in statements of theorems, and it is also used behind the scenes as a technical device in proofs.

## The uniqueness theorems

There are two well-known uniqueness theorems: the gauge-invariant uniqueness theorem and the Cuntz-Krieger uniqueness theorem. The uniqueness theorems are fundamental results in the study of graph C*-algebras, and they serve as cornerstones of the theory. Each provides sufficient conditions for a ${\displaystyle *}$-homomorphism from ${\displaystyle C^{*}(E)}$ into a C*-algebra to be injective. Consequently, the uniqueness theorems can be used to determine when a C*-algebra generated by a Cuntz-Krieger ${\displaystyle E}$-family is isomorphic to ${\displaystyle C^{*}(E)}$; in particular, if ${\displaystyle A}$ is a C*-algebra generated by a Cuntz-Krieger ${\displaystyle E}$-family, the universal property of ${\displaystyle C^{*}(E)}$ produces a surjective ${\displaystyle *}$-homomorphism ${\displaystyle \phi :C^{*}(E)\to A}$, and the uniqueness theorems each give conditions under which ${\displaystyle \phi }$ is injective, and hence an isomorphism. Formal statements of the uniqueness theorems are as follows:

The Gauge-Invariant Uniqueness Theorem: Let ${\displaystyle E}$ be a graph, and let ${\displaystyle C^{*}(E)}$ be the associated graph C*-algebra. If ${\displaystyle A}$ is a C*-algebra and ${\displaystyle \phi :C^{*}(E)\to A}$ is a ${\displaystyle *}$-homomorphism satisfying the following two conditions:

1. there exists a gauge action ${\displaystyle \beta :\mathbb {T} \to \operatorname {Aut} A}$ such that ${\displaystyle \phi \circ \beta _{z}=\gamma _{z}\circ \phi }$ for all ${\displaystyle z\in \mathbb {T} }$, where ${\displaystyle \gamma }$ denotes the canonical gauge action on ${\displaystyle C^{*}(E)}$, and
2. ${\displaystyle \phi (p_{v})\neq 0}$ for all ${\displaystyle v\in E^{0}}$,

then ${\displaystyle \phi }$ is injective.

The Cuntz-Krieger Uniqueness Theorem: Let ${\displaystyle E}$ be a graph satisfying Condition (L), and let ${\displaystyle C^{*}(E)}$ be the associated graph C*-algebra. If ${\displaystyle A}$ is a C*-algebra and ${\displaystyle \phi :C^{*}(E)\to A}$ is a ${\displaystyle *}$-homomorphism with ${\displaystyle \phi (p_{v})\neq 0}$ for all ${\displaystyle v\in E^{0}}$, then ${\displaystyle \phi }$ is injective.

The gauge-invariant uniqueness theorem implies that if ${\displaystyle \{s_{e},p_{v}:e\in E^{1},v\in E^{0}\}}$ is a Cuntz-Krieger ${\displaystyle E}$-family with nonzero projections and there exists a gauge action ${\displaystyle \beta }$ with ${\displaystyle \beta _{z}(p_{v})=p_{v}}$ and ${\displaystyle \beta _{z}(s_{e})=zs_{e}}$ for all ${\displaystyle v\in E^{0}}$, ${\displaystyle e\in E^{1}}$, and ${\displaystyle z\in \mathbb {T} }$, then ${\displaystyle \{s_{e},p_{v}:e\in E^{1},v\in E^{0}\}}$ generates a C*-algebra isomorphic to ${\displaystyle C^{*}(E)}$. The Cuntz-Krieger uniqueness theorem shows that when the graph satisfies Condition (L) the existence of the gauge action is unnecessary; if a graph ${\displaystyle E}$ satisfies Condition (L), then any Cuntz-Krieger ${\displaystyle E}$-family with nonzero projections generates a C*-algebra isomorphic to ${\displaystyle C^{*}(E)}$.

## Ideal structure

The ideal structure of ${\displaystyle C^{*}(E)}$ can be determined from ${\displaystyle E}$. A subset of vertices ${\displaystyle H\subseteq E^{0}}$ is called hereditary if for all ${\displaystyle e\in E^{1}}$, ${\displaystyle s(e)\in H}$ implies ${\displaystyle r(e)\in H}$. A hereditary subset ${\displaystyle H}$ is called saturated if whenever ${\displaystyle v}$ is a regular vertex with ${\displaystyle s^{-1}(v)\subseteq H}$, then ${\displaystyle v\in H}$. The saturated hereditary subsets of ${\displaystyle E}$ are partially ordered by inclusion, and they form a lattice with meet ${\displaystyle H_{1}\wedge H_{2}:=H_{1}\cap H_{2}}$ and join ${\displaystyle H_{1}\vee H_{2}}$ defined to be the smallest saturated hereditary subset containing ${\displaystyle H_{1}\cup H_{2}}$.

If ${\displaystyle H}$ is a saturated hereditary subset, ${\displaystyle I_{H}}$ is defined to be closed two-sided ideal in ${\displaystyle C^{*}(E)}$ generated by ${\displaystyle \{p_{v}:v\in H\}}$. A closed two-sided ideal ${\displaystyle I}$ of ${\displaystyle C^{*}(E)}$ is called gauge invariant if ${\displaystyle \gamma _{z}(a)\in C^{*}(E)}$ for all ${\displaystyle a\in I}$ and ${\displaystyle z\in \mathbb {T} }$. The gauge-invariant ideals are partially ordered by inclusion and form a lattice with meet ${\displaystyle I_{1}\wedge I_{2}:=I_{1}\cap I_{2}}$ and joint ${\displaystyle I_{1}\vee I_{2}}$ defined to be the ideal generated by ${\displaystyle I_{1}\cup I_{2}}$. For any saturated hereditary subset ${\displaystyle H}$, the ideal ${\displaystyle I_{H}}$ is gauge invariant.

The following theorem shows that gauge-invariant ideals correspond to saturated hereditary subsets.

Theorem: Let ${\displaystyle E}$ be a row-finite graph. Then the following hold:

1. The function ${\displaystyle H\mapsto I_{H}}$ is a lattice isomorphism from the lattice of saturated hereditary subsets of ${\displaystyle E}$ onto the lattice of gauge-invariant ideals of ${\displaystyle C^{*}(E)}$ with inverse given by ${\displaystyle I\mapsto \{v\in E^{0}:p_{v}\in I\}}$.
2. For any saturated hereditary subset ${\displaystyle H}$, the quotient ${\displaystyle C^{*}(E)/I_{H}}$ is ${\displaystyle *}$-isomorphic to ${\displaystyle C^{*}(E\setminus H)}$, where ${\displaystyle E\setminus H}$ is the subgraph of ${\displaystyle E}$ with vertex set ${\displaystyle (E\setminus H)^{0}:=E^{0}\setminus H}$ and edge set ${\displaystyle (E\setminus H)^{1}:=E^{1}\setminus r^{-1}(H)}$.
3. For any saturated hereditary subset ${\displaystyle H}$, the ideal ${\displaystyle I_{H}}$ is Morita equivalent to ${\displaystyle C^{*}(E_{H})}$, where ${\displaystyle E_{H}}$ is the subgraph of ${\displaystyle E}$ with vertex set ${\displaystyle E_{H}^{0}:=H}$ and edge set ${\displaystyle E_{H}^{1}:=s^{-1}(H)}$.
4. If ${\displaystyle E}$ satisfies Condition (K), then every ideal of ${\displaystyle C^{*}(E)}$ is gauge invariant, and the ideals of ${\displaystyle C^{*}(E)}$ are in one-to-one correspondence with the saturated hereditary subsets of ${\displaystyle E}$.

## K-theory

The K-groups of a graph C*-algebra may be computed entirely in terms of information coming from the graph. If ${\displaystyle E}$ is a row-finite graph, the vertex matrix of ${\displaystyle E}$ is the ${\displaystyle E^{0}\times E^{0}}$ matrix ${\displaystyle A_{E}}$ with entry ${\displaystyle A_{E}(v,w)}$ defined to be the number of edges in ${\displaystyle E}$ from ${\displaystyle v}$ to ${\displaystyle w}$. Since ${\displaystyle E}$ is row-finite, ${\displaystyle A_{E}}$ has entries in ${\displaystyle \mathbb {N} \cup \{0\}}$ and each row of ${\displaystyle A_{E}}$ has only finitely many nonzero entries. (In fact, this is where the term "row-finite" comes from.) Consequently, each column of the transpose ${\displaystyle A_{E}^{t}}$ contains only finitely many nonzero entries, and we obtain a map ${\displaystyle A_{E}^{t}:\bigoplus _{E^{0}}\mathbb {Z} \to \bigoplus _{E^{0}}\mathbb {Z} }$ given by left multiplication. Likewise, if ${\displaystyle I}$ denotes the ${\displaystyle E^{0}\times E^{0}}$ identity matrix, then ${\displaystyle I-A_{E}^{t}:\bigoplus _{E^{0}}\mathbb {Z} \to \bigoplus _{E^{0}}\mathbb {Z} }$ provides a map given by left multiplication.

Theorem: Let ${\displaystyle E}$ be a row-finite graph with no sinks, and let ${\displaystyle A_{E}}$ denote the vertex matrix of ${\displaystyle E}$. Then

${\displaystyle I-A_{E}^{t}:\bigoplus _{E^{0}}\mathbb {Z} \to \bigoplus _{E^{0}}\mathbb {Z} }$

gives a well-defined map by left multiplication. Furthermore,

${\displaystyle K_{0}(C^{*}(E))\cong \operatorname {coker} (I-A_{E}^{t})\qquad {\text{and}}\qquad K_{1}(C^{*}(E))\cong \operatorname {ker} (I-A_{E}^{t})}$.

In addition, if ${\displaystyle C^{*}(E)}$ is unital (or, equivalently, ${\displaystyle E^{0}}$ is finite), then the isomorphism ${\displaystyle K_{0}(C^{*}(E))\cong \operatorname {coker} (I-A_{E}^{t})}$ takes the class of the unit in ${\displaystyle K_{0}(C^{*}(E))}$ to the class of the vector ${\displaystyle (1,1,\ldots ,1)}$ in ${\displaystyle \operatorname {coker} (I-A_{E}^{t})}$.

Since ${\displaystyle K_{1}(C^{*}(E))}$ is isomorphic to a subgroup of the free group ${\displaystyle \bigoplus _{E^{0}}\mathbb {Z} }$, we may conclude that ${\displaystyle K_{1}(C^{*}(E))}$ is a free group. It can be shown that in the general case (i.e., when ${\displaystyle E}$ is allowed to contain sinks or infinite emitters) that ${\displaystyle K_{1}(C^{*}(E))}$ remains a free group. This allows one to produce examples of C*-algebras that are not graph C*-algebras: Any C*-algebra with a non-free K1-group is not Morita equivalent (and hence not isomorphic) to a graph C*-algebra.

## Notes

1. ^ 2004 NSF-CBMS Conference on Graph Algebras [1]
2. ^ NSF Award [2]
3. ^ a b Kumjian, Alex; Pask, David; Raeburn, Iain Cuntz-Krieger algebras of directed graphs. Pacific J. Math. 184 (1998), no. 1, 161–174.
4. ^ Bates, Teresa; Pask, David; Raeburn, Iain; Szymański, Wojciech The C*-algebras of row-finite graphs. New York J. Math. 6 (2000), 307–324.
5. ^ Raeburn, Iain Graph algebras. CBMS Regional Conference Series in Mathematics, 103. Published for the Conference Board of the Mathematical Sciences, Washington, DC; by the American Mathematical Society, Providence, RI, 2005. vi+113 pp. ISBN 0-8218-3660-9
6. ^ D. Drinen,Viewing AF-algebras as graph algebras, Proc. Amer. Math. Soc., 128 (2000), pp. 1991–2000.