  We consider a hydrodynamic model of self-organized evolution of agents, with
singular interaction kernel $\phi_\alpha(x)=1/|x|^{1+\alpha}$ ($0<\alpha<2$),
in the presence of an additional external force. Well-posedness results are
already available for the unforced system in classical regularity spaces. We
define a notion of solution in larger function spaces, in particular in
$L^\infty$ ("weak solutions") and in $W^{1,\infty}$ ("strong solutions"), and
we discuss existence and uniqueness of these solutions. Furthermore, we show
that several important properties of classical solutions carry over to these
less regular ones. In particular, we give Onsager-type criteria for the
validity of the natural energy law for weak solutions of the system, and we
show that fast alignment (weak and strong solutions) and flocking (strong
solutions) still occur in the forceless case.
