In this paper, we prove a Kastler-Kalau-Walze type theorem for perturbations of Dirac operators on compact manifolds with or without boundary. As a corollary, we give two kinds of operator-theoretic explanations of the gravitational action on boundary. We also compute the spectral action for Dirac operators with two-form perturbations on 4-dimensional compact manifolds.
Perturbations ofDirac operators were investigated by several authors. In [SZ], Sitarz and Zajac investigated the spectral action for scalar perturbations of Dirac operators. In [IL,p.305], Iochum and Levy computed the heat kernel coefficients for Dirac operators with one-form perturbations. In [HPS], Hanisch, Pfäffle and Stephan derived a formula for the gravitational part of the spectral action for Dirac operators on 4-dimensional spin manifolds with totally anti-symmetric torsion and this is a perturbation with three-forms of Dirac operators. On the other hand, in [CM], Connes and Moscovici considered the conformal perturbations of Dirac operators. Investigating the perturbations of Dirac operators has some significance (see [IL],[HPS],[CC]). Motivated by [IL], [SZ] and [HPS], we study the Dirac operators with general form perturbations. We prove a Kastler-Kalau-Walze type theorem for general form perturbations and the conformal perturbations of Dirac operators for compact manifolds with or without boundary. We also compute the spectral action for Dirac operators with two-form perturbations on 4-dimensional compact manifolds and give detailed computations of spectral action for scalar perturbations of Dirac operators in [SZ].This paper is organized as follows. In Section 2, we prove the Lichnerowicz formula for perturbations of Dirac operators and prove a Kastler-Kalau-Walze type theorem for perturbations of Dirac operators on 4-dimensional compact manifolds with or without boundary. In Section 3, we prove a Kastler-Kalau-Walze type theorem for conformal perturbations of Dirac operators on compact manifolds with or without boundary. In Section 4, We compute the spectral action for Dirac operators with scalar and two-form perturbations on 4-dimensional compact manifolds.