  In this paper we study phase transitions for weakly interacting multiagent
systems. Using linear response theory we probe the emergence of singular
behaviour in the thermodynamic limit by studying the properties of the real and
imaginary parts of the susceptibility. We find clear evidence of the loss of
analyticity due to a pole crossing the real axis of frequencies. Such behaviour
has a high degree of universality, as it does not depend on either the applied
forcing or on the considered observable. We present results relevant for both
equilibrium and nonequilibrium phase transitions by studying the Desai-Zwanzig
and Bonilla-Casado-Morillo models.
