Geodesic
Trace and Differences of Idempotents in $C^*$-Algebras
Matematičeskie zametki, Tome 105 (2019) no. 5, pp. 647-655.

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Let $\varphi$ be a trace on a unital $C^*$-algebra $\mathcal{A}$, let $\mathfrak{M}_{\varphi}$ be the ideal of definition of the trace $\varphi$, and let $P,Q \in \mathcal{A}$ be idempotents such that $QP=P$. If $Q \in \mathfrak{M}_{\varphi}$, then $P \in \mathfrak{M}_{\varphi}$ and $0 \le \varphi(P) \le \varphi(Q)$. If $Q-P \in \mathfrak{M}_{\varphi}$, then $\varphi(Q-P)\in \mathbb{R}^+$. Let $A,B\in \mathcal{A}$ be tripotents. If $AB=B$ and $A\in \mathfrak{M}_{\varphi}$, then $B \in \mathfrak{M}_{\varphi}$ and $0 \le \varphi (B^2)\le \varphi (A^2)+\infty$. Let $\mathcal{A}$ be a von Neumann algebra. Then $$ \varphi(|PQ-QP|)\le \min\{\varphi(P),\varphi(Q),\varphi(|P-Q|)\} $$ for all projections $P,Q \in \mathcal{A}$. The following conditions are equivalent for a positive normal functional $\varphi$ on a von Neumann algebra $\mathcal{A}$: (i) $\varphi $ is a trace; (ii) $\varphi(Q-P) \in \mathbb{R}^+$ for all idempotents $P,Q \in \mathcal{A}$ with $QP=P$; (iii) $ \varphi(|PQ-QP|) \le \min\{\varphi(P),\varphi(Q)\}$ for all projections $P,Q \in \mathcal{A}$; (iv) $\varphi(PQ+QP) \le \varphi(PQP+QPQ)$ for all projections $P,Q \in \mathcal{A}$.
Keywords: Hilbert space, linear operator, idempotent, projection, trace-class operators, commutator, von Neumann algebra, $C^*$-algebra, trace.
Mots-clés : tripotent 
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A. M. Bikchentaev. Trace and Differences of Idempotents in $C^*$-Algebras. Matematičeskie zametki, Tome 105 (2019) no. 5, pp. 647-655. http://geodesic.mathdoc.fr/item/MZM_2019_105_5_a0/

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