Consider the Green's function of fermion operators with imaginary time, $$\mathcal{G}(\nu, \nu', \tau) = - \langle T_\tau c_{\nu}(\tau) c_{\nu'}^\dagger(0)\rangle\tag{1}$$ To show it satisfies the periodicity, $$\mathcal{G}(\nu, \nu', \tau) = - \mathcal{G}(\nu, \nu', \tau+ \beta) \tag{2}$$ one needs to used the following identity, (see eq (71) and (72) on page 16 of http://folk.ntnu.no/johnof/green-2013.pdf) $${\rm Tr}(ABC \ldots XY Z) = {\rm Tr}(ZAB \ldots XY )\tag{71}$$
$$Tr(e^{-\beta H}c_{\nu'}^\dagger e^{H \tau} c_\nu e^{-H \tau} ) = Tr( e^{H \tau} c_\nu e^{-H \tau} e^{-\beta H}c_{\nu'}^\dagger )\tag{3}$$ which is important for the appearance of the minus sign in equation (2). However, since $c_\nu$ and $c_\nu^\dagger$ are fermions, I doubt equation (3) is not correct. My question is whether (3) is indeed correct or not? If (3) is not correct, how can (2) hold?