Business Cycle and Conserved Quantity in Economics
  We propose a dynamical model for business cycle based on an optimal DI model. In the model there exists a conserved quantity, which corresponds to the total energy in a dynamical system. We found that the business cycle with the period 6 or 7 years is nicely reproduced, since the model predicts a periodic motion in the conservative system. 
Masa-aki Taniguchi, Masako Bando, and Akihiro Nakayama