My textbook defines the condition for a system's stability as amp being less than 0 dB on the frequency where phase shift is -180°. Then it gives me a transfer function 1/s and asks whether it is stable or not. The phase shift of that function is -90° at every (positive) frequency so I can't exactly apply the given condition to it, so is the transfer function stable or unstable? It also asks whether or not -1/s is stable.