Combinatorial Algebra for Normed Structures

Transcription

1 Combinatorial Algebra for Normed Structures Will Grilliette Alfred University Recent Progress in Operator Algebras, 1074 th AMS Meeting

2 Scaled-Free Objects Normed Sets Traditionally [1, 3, 7], universal C*-algebras have been constructed by building a complex *-algebra on a set S subject to certain *-algebraic relations R, and then norming by certain representation restrictions. C S C S R S R 1C However, this is counter to the algebraic means of quotienting a free object.

3 Scaled-Free Objects Normed Sets Proposition (Folklore) Let S and C any subcategory of normed vector spaces with contractive maps. If Ob(C ) contains V = O, then S has no associated free object in C. The classical notions can be recovered by replacing Set. Definition ([3, 4]) A normed set is a pair (S, f ), where S is a set and f a function from S to [0, ). Given two normed sets (S, f ) and (T, g), a function φ : S T is contractive if g (φ(s)) f (s) for all s S.

4 Scaled-Free Objects Outline of Construction Given a normed set (S, f ), 1 Form the set S f := S \ f 1 (0). 2 Construct the free unital *-algebra A S,f over C on S f. 3 Construct a C*-norm on A S,f from f. 4 Complete A S,f into a unital C*-algebra A S,f. Theorem (Scaled-Free Mapping Property, [6]) Let (S, f ) be a normed set, B a unital C*-algebra, and φ : (S, f ) B a function. Then, there is a unique unital *-homomorphism ˆφ : A S,f B such that for all s S. φ(s) B ˆφ(s) = f (s) φ(s)

5 Scaled-Free Objects Definitions Definition A C*-relation on (S, f ) is an element of A S,f. Definition For a crutched set (S, f ) and C*-relations R A S,f on (S, f ), let J R be the two-sided, norm-closed ideal generated by R in A S,f. Then, the unital C*-algebra presented on (S, f ) subject to R is S, f R 1C := A S,f /J R. C S C S R S, f 1C S, f R 1C Fact: This square commutes for all *-algebraic relations R.

6 Tietze Transformations The Transformations For group theory, Tietze ([9], 1908) described canonical means of converting between presentations of the same group. These same transformations exist for this presentation theory for 1C. 1 Adding/Removing C*-relations. (e.g. (x, λ) x = x 2 1C (x, λ) x = x 2, x = x 5 1C ) 2 Adding/Removing generators. (e.g. (x, λ) x = x 2 1C (x, λ), ( y, λ 2) x = x 2, y = x x 1C ) One of these transformations is elementary if only one generator or C*-relation is altered.

7 Tietze Transformations Examples Consider the C*-algebra of a left-invertible element. L := (x, λ) µ 2 x x 1 1C. If λµ < 1, 1 L < 1. Thus, 1 = 0 so L = 1C O.

8 Tietze Transformations Examples For λµ 1, L = 1C = 1C = 1C (x, λ), (q, λ), (u, λµ) (x, λ), (q, λ), (u, λµ) (x, λ),(q, λ), (u, λµ) µ 2 x x 1, q = (x x) 1 2, ( ) ) u = µx p (µ (x x) µ 2 x x 1, q = (x x) 1 2, ( ) u = µx p (µ (x x) ), 1 µq, u u = 1, x = uq 1 µq, u u = 1, x = uq 1C 1C 1C = 1C (q, λ), (u, λµ) 1 µq, u u = 1 1C = 1C (q, λ) 1 µq 1C C (u, λµ) u u = 1 1C = 1C C [ ] 1 µ, λ C T

9 Tietze Transformations Examples Consider the C*-algebra of a single idempotent. A := (x, λ) x = x 2 1C. If λ < 1, then x = 0. Hence, A = 1C C. For λ 1, the range and kernel projections can be formed from x, [2, Proposition IV.1.1]. Likewise, x can be written in terms of these projections, [10, Theorem 1]. A = 1C (r, 1), (k, 1) r 2 = r = r, k 2 = k = k, rk 1 λ 2 By [8, Theorem 3.2], A = 1C C 2, λ = 1, [ C[0, 1] ] C0 (0, 1] C 0 (0, 1] C[0, 1], λ > 1. 1

10 Tietze Transformations Main Theorem Theorem (Tietze Theorem for 1C, [5]) Given unital C*-algebras A and B, A = 1C B iff there is a sequence of four Tietze transformations changing the presentation of A into the presentation for B. Corollary (Elementary Version, [5]) Given finitely presented unital C*-algebras A and B, A = 1C B iff there is a finite sequence of elementary Tietze transformations changing the presentation of A into the presentation for B.

11 Further Study Analytic/continuous relations (sin(x) = 0, etc.) Formalize familiar universal constructions. (free product, tensor product, etc.) Characterization of properties. (projectivity, separability, etc.) Other categories of interest (Banach algebras, operator algebras, etc.)

12 Bibliography Bruce Blackadar. Shape theory for C -algebras. Math. Scand., 56(2): , Kenneth R. Davidson. C -algebras by example, volume 6 of Fields Institute Monographs. American Mathematical Society, Providence, RI, Eberhard H.-A. Gerbracht. Elemente einer kombinatorischen Theorie der C*-Algebren: Präsentationen von C*-Algebren mittels Erzeugender und Relationen. PhD thesis, Technischen Universität Carolo-Wilhelmina zu Braunschweig, March 1998.

13 Bibliography Marco Grandis. Normed combinatorial homology and noncommutative tori. Theory Appl. Categ., 13:No. 7, , Will Grilliette. Presentations and tietze transformations of c*-algebras , v2. Will Grilliette. Scaled-free objects , v2. Terry A. Loring. Lifting solutions to perturbing problems in C -algebras, volume 8 of Fields Institute Monographs. American Mathematical Society, Providence, RI, 1997.

14 Bibliography Gert Kjaergȧrd Pedersen. Measure theory for C algebras. II. Math. Scand., 22:63 74, Heinrich Tietze. Über die topologischen Invarianten mehrdimensionaler Mannigfaltigkeiten. Monatsh. Math. Phys., 19(1):1 118, Ivan Vidav. On idempotent operators in a Hilbert space. Publ. Inst. Math. (Beograd) (N.S.), 4 (18): , 1964.