The Cosmological Constant as a Function of Extrinsic Curvature and
  Spatial Curvature
  In this paper we suppose that the cosmological constant will change when the
universe expends. For a general consideration, the cosmological constant is
assumed to be a function of scale factor and Hubble constant. According to the
ADM formulation, to the FRW metric, the extrinsic curvature $I$ equals
$-6H^{2}$ and spatial curvature $R$ equals $6k/a^{2}$. Therefore we suppose
cosmological constant is a function of extrinsic curvature and spatial
curvature. We investigate the cosmological evolution of FRW universe in these
models. At last we investigate two particular models which could fit the
observation data about dark energy well. Actually a changeless cosmological
constant is not essential. If when the universe expands, the cosmological
constant changes slowly and gradually flows to a constant, the observation data
about dark energy could also be fitted well by this kind of theory.
