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Algebras of operators on Hilbert space, $C^*-$algebras, von Neumann algebras, non-commutative geometry

6 votes

Representation of $*$-automorphism on finite dimensional matrix algebras

If $\phi$ is a $*$-automorphism then $\psi:A\mapsto\phi(\overline A)$ is a $\mathbb{C}$-automorphism. By the Skolem-Noether theorem every $\mathbb{C}$-automorphism of $M_n(\mathbb{C})$ is inner, that …
Robin Chapman's user avatar
25 votes

Commutative subalgebras of M_n

See the related question at Dimension of subalgebras of a matrix algebra . In particular, I'd recommend the reference: M. Mirzakhani `A simple proof of a theorem of Schur' Amer. Math. Monthly 105 (199 …
Robin Chapman's user avatar
5 votes

Commutative subalgebras of M_n

This is a reply to Tom's reply. Let's stick to commutative subalgebras of $M_n(k)$ where $k$ is algebraically closed. Let $A$ be a unital commutative subalgebra of $M_n(k)$. Then $N=k^n$ is a faithful …
Robin Chapman's user avatar
3 votes
Accepted

Intersection of ideals in C*-algebra or even rings in general

In the most general form, for arbitrary ideals over rings, this is false. In the ring $\mathbb{Z}$ let $I_k$ be generated by $2^k$ and let $J$ be generated by $3$. Then $I_k+J=\mathbb{Z}$ for all $k$ …
Robin Chapman's user avatar