In mathematics, a universal C*-algebra is a C*-algebra described in terms of generators and relations. In contrast to rings or algebras, where one can consider quotients by free rings to construct universal objects, C*-algebras must be realizable as algebras of bounded operators on a Hilbert space by the Gelfand-Naimark-Segal construction and the relations must prescribe a uniform bound on the norm of each generator. This means that depending on the generators and relations, a universal C*-algebra may not exist. In particular, free C*-algebras do not exist.
There are several problems with defining relations for C*-algebras. One is, as previously mentioned, due to the non-existence of free C*-algebras, not every set of relations defines a C*-algebra. Another problem is that one would often want to include order relations, formulas involving continuous functional calculus, and spectral data as relations. For that reason, we use a relatively roundabout way of defining C*-algebra relations. The basic motivation behind the following definitions is that we will define relations as the category of their representations.
Given a set X, the null C*-relation on X is the category with objects consisting of pairs (j, A), where A is a C*-algebra and j is a function from X to A and with morphisms from (j, A) to (k, B) consisting of *-homomorphisms φ from A to B satisfying φ ∘ j = k. A C*-relation on X is a full subcategory of satisfying:
Given a C*-relation R on a set X. then a function ι from X to a C*-algebra U is called a universal representation for R if
A C*-relation R has a universal representation if and only if R is compact.
Given a *-polynomial p on a set X, we can define a full subcategory of with objects (j, A) such that p ∘ j = 0. For convenience, we can call p a relation, and we can recover the classical concept of relations. Unfortunately, not every *-polynomial will define a compact C*-relation.
Alternatively, one can use a more concrete characterization of universal C*-algebras that more closely resembles the construction in abstract algebra. Unfortunately, this restricts the types of relations that are possible. Given a set G, a relation on G is a set R consisting of pairs (p, η) where p is a *-polynomial on X and η is a non-negative real number. A representation of (G, R) on a Hilbert space H is a function ρ from X to the algebra of bounded operators on H such that for all (p, η) in R. The pair (G, R) is called admissible if a representation exists and the direct sum of representations is also a representation. Then
is finite and defines a seminorm satisfying the C*-norm condition on the free algebra on X. The completion of the quotient of the free algebra by the ideal is called the universal C*-algebra of (G,R).
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