Covariant perturbation expansion of off-diagonal heat kernel
  Covariant perturbation expansion is an important method in quantum field theory. In this paper an expansion up to arbitrary order for off-diagonal heat kernels in flat space based on the covariant perturbation expansion is given. In literature, only diagonal heat kernels are calculated based on the covariant perturbation expansion. 
Yu-Zi Gou, Wen-Du Li, Ping Zhang, and Wu-Sheng Dai
cond-mat.stat-mech hep-th math-ph math.MP quant-ph