  Let $A(t)$ be a continuous path of Fredhom operators, we first prove that the
spectral flow $sf(A(t))$ is cogredient invariant. Based on this property, we
give a decomposition formula of spectral flow if the path is invariant under a
matrix-like cogredient. As applications, we give the generalized Bott-type
iteration formula for linear Hamiltonian systems.
