New tensor products of C*-algebras and characterization of type I C*-algebras as rigidly symmetric C*-algebras
For C*-algebras A and B, we construct three related classes of cross-norm completions A⊗μB of A⊙B. The first class of these constructions produces Banach algebras, the second Banach ∗-algebras, and the third produces C*-algebras. For certain discrete groups G1 and G2, such as noncommutative free groups, our C*-completions of C∗r(G1)⊙C∗r(G2) coincide with a Brown-Guentner type C*-completion of ℓ1(G1×G2) and produces 2ℵ0 distinct C*-completions of C∗r(G1)⊙C∗r(G2). A Banach ∗-algebra A is symmetric if the spectrum σ(a∗a) is contained in [0,∞) for every a∈A and rigidly symmetric if the Banach space tensor product A⊗γB is symmetric for every C*-algebra B. A theorem of Kügler asserts that every type I C*-algebra is rigidly symmetric. Leveraging our new constructions, we establish the converse of Kügler's theorem by showing for C*-algebras A and B that A⊗γB is symmetric if and only if A or B is type I. This strongly settles a question of Leptin and Poguntke from 1979.