  Using oblique projections and angles between subspaces we write condition
number estimates for abstract nonsymmetric domain decomposition methods. In
particular, we consider a restricted additive method for the Poisson equation
and write a bound for the condition number of the preconditioned operator. We
also obtain the non-negativity of the preconditioned operator. Condition number
estimates are not enough for the convergence of iterative methods such as GMRES
but these bounds may lead to further understanding of nonsymmetric domain
decomposition methods.
