  In analogy to Neuberger's double-pass algorithm for the Conjugate Gradient
inversion with multi-shifts we introduce a double-pass variant for
BiCGstab(ell). One possible application is the overlap operator of QCD at
non-zero chemical potential, where the kernel of the sign function is
non-Hermitian. The sign function can be replaced by a partial fraction
expansion, requiring multi-shift inversions. We compare the performance of the
new method with other available algorithms, namely partial fraction expansions
with restarted FOM inversions and the Krylov-Ritz method using nested Krylov
subspaces.
