A C*-algebra is Rickart if for each $x\in A$ there is a projection $p\in A$ so that $R(x)=pA$. Here the right-annihilator $R(S)$ of $S\subset A$ is defined as $$R(S)=\{a\in A\mid xa=0\, \forall x\in S\}$$ and $R(x)\equiv R(\{x\})$.


Kazuyuki Saito and J. D. Maitland Wright. $C^∗$-algebras which are Grothendieck spaces. Rend. Circ. Mat. Palermo (2), 52(1):141–144, 2003.

an alternative definition is studied: define a C*-algebra to be Rickart if each maximal Abelian *-subalgebra of $A$ is Rickart (or, equivalently, monotone $\sigma$-complete). Equivalently, one may require that every Abelian *-subalgebra is contained in an Abelian Rickart C*-algebra.

This definition is more general and seems to be sufficient for many applications.

Is this definition in fact equivalent to the original one?

This question recently came up in our investigations in the foundations of quantum theory:

Bohrification of operator algebras and quantum logic

  • $\begingroup$ Based on the remarks in your paper, this may deserve the "open-problem" tag. $\endgroup$ – Jonas Meyer Jan 14 '10 at 8:57

They are equivalent. See On Defining AW*-algebras and Rickart C*-algebras by K. Saitô, and J.D.M. Wright (arXiv:1501.02434)

  • 1
    $\begingroup$ Oh dear, why down-vote? $\endgroup$ – Tomasz Kania Jan 14 '15 at 11:26
  • $\begingroup$ Welcome to MO, and thanks for filling this gap in the literature! $\endgroup$ – Chris Heunen Jan 14 '15 at 19:54

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