View source for Quantale

{{MSC|06F07}}

A complete sup-lattice $Q$ together with an associative product $\otimes$ satisfying the distributive laws
$$
a \otimes \left({ \bigvee_i b_i }\right) = \bigvee_i a \otimes b_i
$$
$$
\left({ \bigvee_i b_i }\right) \otimes a= \bigvee_i b_i \otimes a
$$
for all $a, b_i \in Q$ (cf. also [[Lattice]]; [[Distributivity]]; [[Associativity]]).

The name  "quantale"  was introduced by C.J. Mulvey [[#References|[a1]]] to provide a non-commutative extension of the concept of [[locale]]. The intention was to develop the concept of non-commutative topology introduced by R. Giles and H. Kummer [[#References|[a2]]], while providing a constructive, and non-commutative, context for the foundations of quantum mechanics and, more generally, non-commutative logic. The observation that the closed right ideals of a [[C*-algebra|$C^*$-algebra]] form a quantale satisfying the conditions that each element is right-sided ($a \otimes 1_Q \le a$) and idempotent ($a \otimes a = a$)) led certain authors to restrict the term  "quantale"  to mean only quantales of this kind [[#References|[a3]]], but the term is now applied only in its original sense.

The realization by J. Rosický [[#References|[a4]]] that the development of topological concepts such as regularity required additional structure led [[#References|[a5]]] to the consideration of involutive quantales, and of the spectrum $\text{Max} A$ of a $C^*$-algebra $A$ (cf. also [[Spectrum of a C*-algebra|Spectrum of a $C^*$-algebra]]) as the quantale of closed linear subspaces of $A$, together with the operations of join given by closed linear sum, product given by closed linear product of subspaces, and involution by involution within the $C^*$-algebra. The right-sided elements of the spectrum $\text{Max} A$ are the closed right ideals of the $C^*$-algebra $A$ (cf. [[#References|[a2]]], [[#References|[a6]]]). By the existence of approximate units, each element $a \in R(\text{Max} A)$ of the sup-lattice of right-sided elements satisfies the condition that $a \otimes a^* \otimes a = a$. By a ''Gel'fand quantale'' $Q$ is meant an involutive unital quantale in which the right-sided (equivalently, left-sided) elements satisfy this condition.

Generalizing an observation in [[#References|[a4]]], the right-sided elements of any involutive quantale $Q$ may be shown to admit a pseudo-orthocomplement, defined by $a^\perp = \bigvee_{a^* \otimes b = 0_Q} b$. In any Gel'fand quantale $Q$, the right-sided elements are idempotent, and the two-sided elements form a locale.

Observing that relations on a set $X$ forming a quantale with respect to arbitrary union and composition is applied implicitly by C.A. Hoare and He Jifeng when considering the weakest pre-specification of a program [[#References|[a7]]], and noting that the quantale $\mathcal{Q}(X)$ in question is exactly that of endomorphisms of the sup-lattice $\mathcal{P}(X)$ of subsets of $X \times X$, led to the consideration [[#References|[a8]]] of the quantale $\mathcal{Q}(S)$ of endomorphisms of any orthocomplemented sup-lattice $S$, in which the involute $\alpha^*$ of a sup-preserving mapping $\alpha$ is defined by $s \alpha^* = \left({ \bigvee_{t \alpha \le s^\perp} t} \right)^\perp$ for each $s \in S$. In the quantale $\mathcal{Q}(X)$ of relations on a set $X$, this describes the [[Transposed relation|reverse]] of a relation in terms of complementation of subsets. Observing that the quantale $\mathcal{Q}(H)$ of endomorphisms of the orthocomplemented sup-lattice of closed linear subspaces of a [[Hilbert space]] $H$ provides a motivating example for this quantization of the calculus of relations, the term ''Hilbert quantale'' was introduced for any quantale isomorphic to the quantale $Q9S)$ of an orthocomplemented sup-lattice $S$.

Noting that the weak spectrum $\text{Max}_{\text{W}}(B)$ of a [[von Neumann algebra]] $B$ is a Gel'fand quantale of which the right-sided elements correspond to the projections of $B$ and on which the right pseudo-orthocomplement corresponds to orthocomplementation of projections, a Gel'fand quantale $Q$ is said to be a ''von Neumann quantale'' if $(a^\perp)^\perp = a$ for any right-sided element $a \in Q$. For any von Neumann quantale $Q$, the locale $I(Q)$ of two-sided elements is a complete [[Boolean algebra]]. Any Hilbert quantale $Q$ is a von Neumann quantale, and a von Neumann quantale $Q$ is a Hilbert quantale exactly if the canonical homomorphism $\mu_Q : Q \rightarrow \mathcal{Q}(R(Q))$, assigning to each $a \in Q$ the sup-preserving mapping $b \in R(Q) \mapsto a^* \otimes b \in R(Q)$ on the orthocomplemented sup-lattice $R(Q)$ of right-sided elements of $Q$, is an isomorphism [[#References|[a8]]]. Any Hilbert quantale $Q$ is a von Neumann factor quantale in the sense that $I(Q)$ is exactly $\mathbf2]$. The weak spectrum $\text{Max}_{\text{W}}(B)$ of a von Neumann algebra $B$ is a factor exactly if $B$ is a factor [[#References|[a9]]] (cf. also [[von Neumann algebra]]).

A homomorphism $\phi : Q \rightarrow \mathcal{Q}(S)$ from a Gel'fand quantale $Q$ to the Hilbert quantale $\mathcal{Q}(S)$ of an orthocomplemented sup-lattice $S$ is said to be a representation of $Q$ on $S$ [[#References|[a10]]]. A representation is said to be ''irreducible'' provided that $s \in S$ invariant (in the sense that $s \phi_a \le s$ for all $a \in Q$) implies $s = 0_Q$ or $s = 1_Q$. The irreducibility of a representation $\phi : Q \rightarrow \mathcal{Q}(S)$ is equivalent to the homomorphism being ''strong'', in the sense that $\phi(1_Q) = 1_{\mathcal{Q}(S)}$. A homomorphism $Q' \rightarrow Q$ of Gel'fand quantales is strong exactly if $Q' \rightarrow Q \rightarrow \mathcal{Q}(S)$ is irreducible whenever $Q \rightarrow \mathcal{Q}(S)$ is irreducible. A representation $\phi : Q \rightarrow \mathcal{Q}(S)$ of $Q$ on an atomic orthocomplemented sup-lattice $S$ is said to be ''algebraically irreducible'' provided that for any atoms $x,y \in S$ there exists an $a \in Q$ such that $x\phi_a = y$ (cf. also [[atomic lattice]]). Any algebraically irreducible representation is irreducible: the algebraically irreducible representations are those for which every atom is a cyclic generator. An algebraically irreducible representation $\phi : Q \rightarrow \mathcal{Q}(S)$ on an atomic orthocomplemented sup-lattice $S$ is said to be a ''point'' of the Gel'fand quantale $Q$. The points of the spectrum $\text{Max} A$ of a $C^*$-algebra $A$ correspond bijectively to the equivalence classes of irreducible representations of $A$ on a Hilbert space [[#References|[a10]]]. (Presently (2000), this is subject to the conjecture that every point of $\text{Max} A$ is non-trivial in the sense that there exists a pure state that maps properly. For a discussion of the role of pure states in this context, see [[#References|[a10]]].) In particular, the spectrum $\text{Max} A$ is an invariant of the $C^*$-algebra $A$. It may be noted that the Hilbert quantale $\mathcal{Q}(S)$ of an atomic orthocomplemented sup-lattice has, to within equivalence, a unique point; moreover, the reflection of such a Gel'fand quantale into the category of locales is exactly $\mathbf{2}$. In particular, the points of any locale are exactly its points in the sense of the theory of locales.

A von Neumann quantale $Q$ is said to be ''atomic'' provided that the orthocomplemented sup-lattice $R(Q)$ of its right-sided elements is atomic. For any atomic von Neumann quantale $Q$ the complete Boolean algebra of two-sided elements $I(Q)$ is atomic. Moreover, the canonical homomorphism $\mu_Q : Q \rightarrow \mathcal{Q}(R(Q))$ is algebraically irreducible exactly if $Q$ is a von Neumann factor quantale. A Gel'fand quantale $Q$ is said to be ''discrete'' provided that it is an atomic von Neumann quantale that admits a central decomposition of the unit $e_Q \in Q$, in the sense that the atoms of the complete Boolean algebra $I(Q)$ majorize a family of central projections with join $e_Q \in Q$. For any atomic von Neumann algebra $B$, the weak spectrum $\text{Max}_{\text{W}} B$ is a discrete von Neumann quantale. A locale $L$ is a discrete von Neumann quantale exactly if it is a complete atomic Boolean algebra, hence the power set of its set of points. A homomorphism $X \rightarrow Q$ of Gel'fand quantales is said to be:

''algebraically strong'' if $X \rightarrow Q \rightarrow \mathcal{Q}(S)$ is algebraically irreducible whenever $Q \rightarrow \mathcal{Q}(S)$ is an algebraically irreducible representation of $Q$ on an atomic orthocomplemented sup-lattice $S$;

a ''right embedding'' if it restricts to an embedding $R(X) \rightarrow R(Q)$ of the lattices of right-sided elements;

''discrete'' if it is an algebraically strong right embedding. 

A Gel'fand quantale $X$ is said to be ''spatial'' if it admits a discrete homomorphism $X \rightarrow Q$ into a discrete von Neumann quantale $Q$ [[#References|[a11]]]. For any $C^*$-algebra $A$, the canonical homomorphism
$$
\text{Max} A \rightarrow \text{Max}_{\text{W}} B
$$
of its spectrum $\text{Max} A$ into the weak spectrum of its enveloping atomic von Neumann algebra $B$ is discrete, hence $\text{Max} A$ is spatial. Similarly, a locale $L$ is spatial as a Gel'fand quantale exactly if its canonical homomorphism into the power set of its set of points is discrete. More generally, a Gel'fand quantale $Q$ is spatial exactly if it has enough points, in the sense that if $a,b \in R(Q)$ are distinct, then there is an algebraically irreducible representation $\phi : Q \rightarrow \mathcal{Q}(S)$ on an atomic orthocomplemented sup-lattice $S$ such that $\phi_a, \phi_b \in R(\mathcal{Q}(S))$ are distinct [[#References|[a11]]].

In other important directions, Girard quantales have been shown [[#References|[a12]]] to provide a semantics for non-commutative linear logic, and Foulis quantales to generalize the Foulis semi-groups of complete orthomodular lattices [[#References|[a13]]]. The concepts of quantal set and of sheaf have been introduced [[#References|[a14]]] for the case of idempotent right-sided quantales, generalizing those for any locale. These concepts may be localized [[#References|[a15]]] to allow the construction of a fibration from which the quantale may be recovered directly. The representation of quantales by quantales of relations has also been examined [[#References|[a16]]].

====References====
<table>
<TR><TD valign="top">[a1]</TD> <TD valign="top">  C.J. Mulvey,   "&amp;"  ''Rend. Circ. Mat. Palermo'' , '''12'''  (1986)  pp. 99–104 {{ZBL|0633.46065}}</TD></TR>
<TR><TD valign="top">[a2]</TD> <TD valign="top">  R. Giles,   H. Kummer,   "A non-commutative generalization of topology"  ''Indiana Univ. Math. J.'' , '''21'''  (1971)  pp. 91–102</TD></TR>
<TR><TD valign="top">[a3]</TD> <TD valign="top">  K.I. Rosenthal,   "Quantales and their applications" , ''Pitman Research Notes in Math.'' , '''234''' , Longman  (1990)</TD></TR>
<TR><TD valign="top">[a4]</TD> <TD valign="top">  J. Rosický,   "Multiplicative lattices and $C^*$-algebras"  ''Cah. Topol. Géom. Diff. Cat.'' , '''30'''  (1989)  pp. 95–110</TD></TR>
<TR><TD valign="top">[a5]</TD> <TD valign="top">  C.J. Mulvey,   "Quantales" , ''Invited Lecture, Summer Conf. Locales and Topological Groups, Curaçao''  (1989)</TD></TR>
<TR><TD valign="top">[a6]</TD> <TD valign="top">  C.A. Akemann,   "Left ideal structure of $C^*$-algebras"  ''J. Funct. Anal.'' , '''6'''  (1970)  pp. 305–317</TD></TR>
<TR><TD valign="top">[a7]</TD> <TD valign="top">  C.A.R. Hoare,   He Jifeng,   "The weakest prespecification"  ''Inform. Proc. Lett.'' , '''24'''  (1987)  pp. 127–132  {{ZBL|0622.68025}}</TD></TR>
<TR><TD valign="top">[a8]</TD> <TD valign="top">  C.J. Mulvey,   J.W. Pelletier,   "A quantisation of the calculus of relations" , ''Category Theory 1991, CMS Conf. Proc.'' , '''13''' , Amer. Math. Soc.  (1992)  pp. 345–360 {{ZBL|0793.06008}}</TD></TR>
<TR><TD valign="top">[a9]</TD> <TD valign="top">  J.W. Pelletier,   "Von Neumann algebras and Hilbert quantales"  ''Appl. Cat. Struct.'' , '''5'''  (1997)  pp. 249–264</TD></TR>
<TR><TD valign="top">[a10]</TD> <TD valign="top">  C.J. Mulvey,   J.W. Pelletier,   "On the quantisation of points"  ''J. Pure Appl. Algebra'' , '''159'''  (2001)  pp. 231–295</TD></TR>
<TR><TD valign="top">[a11]</TD> <TD valign="top">  C.J. Mulvey,   J.W. Pelletier,   "On the quantisation of spaces"  ''J. Pure Appl. Math.''  '''175''' (2002) pp.289-325 {{ZBL|1026.06018}}</TD></TR>
<TR><TD valign="top">[a12]</TD> <TD valign="top">  D. Yetter,   "Quantales and (non-commutative) linear logic"  ''J. Symbolic Logic'' , '''55'''  (1990)  pp. 41–64</TD></TR>
<TR><TD valign="top">[a13]</TD> <TD valign="top">  C.J. Mulvey,   "Foulis quantales"  ''to appear''</TD></TR>
<TR><TD valign="top">[a14]</TD> <TD valign="top">  C.J. Mulvey,   M. Nawaz,   "Quantales: Quantal sets" , ''Non-Classical Logics and Their Application to Fuzzy Subsets: A Handbook of the Mathematical Foundations of Fuzzy Set Theory'' , Kluwer Acad. Publ.  (1995)  pp. 159–217</TD></TR>
<TR><TD valign="top">[a15]</TD> <TD valign="top">  U. Berni-Canani,   F. Borceux,   R. Succi-Cruciani,   "A theory of quantale sets"  ''J. Pure Appl. Algebra'' , '''62'''  (1989)  pp. 123–136</TD></TR>
<TR><TD valign="top">[a16]</TD> <TD valign="top">  C. Brown,   D. Gurr,   "A representation theorem for quantales"  ''J. Pure Appl. Algebra'' , '''85'''  (1993)  pp. 27–42</TD></TR>
</table>

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