Improving Inversions of the Overlap Operator

where ρ ≥ 1 is a mass parameter corresponding to a bare fermion mass mb = m(ρ − 1), and Q is the hermitian Wilson operator with a negative mass parameter −m. Note that γ5 sign(Q) is unitary. We can also define a hermitian overlap operator Dh = γ5Du. Both operators are normal, i.e. they commute with their adjoints. We will approximate the matrix sign function using the Zolotarev partial fraction expansion (ZPFE),