Finite-dimensional subalgebras of \$C^\star\$-algebras - MathOverflow most recent 30 from http://mathoverflow.net 2013-06-20T02:10:35Z http://mathoverflow.net/feeds/question/35207 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/35207/finite-dimensional-subalgebras-of-c-star-algebras Finite-dimensional subalgebras of \$C^\star\$-algebras Andreas Thom 2010-08-11T09:49:03Z 2010-08-14T17:31:34Z <p>Let \$A\$ be a unital \$C^\star\$-algebra and let \$a_1,\dots,a_n\$ be a finite list of normal elements in \$A\$ which (together with their adjoints) generate a norm-dense \$\star\$-subalgebra \$B \subset A\$. Clearly, if \$A\$ is finite-dimensional, then every element in \$A\$ (and hence \$B\$) has finite spectrum. I am asking for the converse.</p> <blockquote> <p><strong>Question:</strong> Assume that each element of \$B\$ has finite spectrum. Is it true that \$A\$ has to be finite-dimensional?</p> </blockquote> <p>The existence of finitely-generated infinite torsion groups shows that this might be a highly non-trivial problem. In this case one would consider the reduced group \$C^\star\$-algebra and note that all monomials in the generators of the group and their inverses (which are equal to the adjoints of the generators) would have finite spectrum. However, the generated algebra would still be infinite-dimensional. I do not know of any simpler way to come up with such an example. In this case it is conceivable that the random-walk operator associated with the generating set (which is an element in the real group ring) has infinite spectrum, even though I did not prove this.</p> <p>Maybe there is also need to consider the spectra of elements in matrices over \$B\$ (which of course follow to be finite if \$A\$ is finite-dimensional.) In view of this, I am not only asking for an answer to the question but also for the right question (or a better one) if the answer to the original question is negative.</p> <p>A stronger assumption would be to assume that \$A\$ itself consists only of elements with finite spectrum. This case seems much easier to approach and the answer seems to be positive. In fact every infinite-dimensional \$C^\star\$-algebra should contain an element with infinite spectrum.</p> <p>Just to get started, a more concrete instance of the question above is:</p> <blockquote> <p><strong>Question:</strong> Let \$p_1, p_2\$ and \$p_3\$ be three projections in a \$C^*\$-algebra with the property that every (non-commutative) polynomial in \$p_1, p_2\$ and \$p_3\$ has finite spectrum. Is it true that the projections generate a finite-dimensional algebra?</p> </blockquote>