Strongly semihereditary rings and rings with dimension

Abstract

The existence of a well-behaved dimension of a finite von Neumann algebra (see [19]) has lead to the study of such a dimension of finite Baer *-rings (see [26]) that satisfy certain *-ring axioms (used in [9]). This dimension is closely related to the equivalence relation $\sim^{*}$ on projections defined by $p\sim^{*} q$ iff $p=xx^*$ and $q=x^*x$ for some $x.$ However, the equivalence $\sim^{a}$ on projections (or, in general, idempotents) defined by $p\sim^{a} q$ iff $p=xy$ and $q=yx$ for some $x$ and $y,$ can also be relevant. There were attempts to unify the two approaches (see [10]). In this work, our agenda is three-fold: (1) We study assumptions on a ring with involution that guarantee the existence of a well-behaved dimension defined for any general equivalence relation on projections $\sim.$ (2) By interpreting $\sim$ as $\sim^{a},$ we prove the existence of a well-behaved dimension of strongly semihereditary rings with a positive definite involution. This class is wider than the class of finite Baer *-rings with dimension considered in the past: it includes some non Rickart *-rings. Moreover, none of the *-ring axioms from [9] and [26] are assumed. (3) As the first corollary of (2), we obtain dimension of noetherian Leavitt path algebras over positive definite fields. Secondly, we obtain dimension of a Baer *-ring $R$ satisfying the first seven axioms from [26] (in particular, dimension of finite $AW^*$-algebras). Assuming the eight axiom as well, $R$ has dimension for $\sim^{*}$ also and the two dimensions coincide.