Phelps also worked with Luigi Paganetto at the University of Rome Tor Vergata and , between 1988-98 , they held the Villa Mondragone International Seminar . ( Structural Slumps : The Modern Equilibrium Theory of Employment , Interest and Assets '' ( see Selected PublicationsPhelps also collaborated closely with Luigi Paganetto at the University of Rome Tor Vergata and , between 1988 â `` 98 , as co-organizers of the Villa Mondragone International Seminar .